Electrolytic solution, electrochemical device, secondary cell, and module

ABSTRACT

An electrolyte solution contains tris(trimethylsilyl) phosphite and at least one fluorinated saturated cyclic carbonate (1) selected from pentafluoropropylethylene carbonate and heptafluoroisobutylethylene carbonate. Also disclosed is an electrochemical device including the electrolyte solution.

TECHNICAL FIELD

The invention relates to electrolyte solutions, electrochemical devices,secondary batteries, and modules.

BACKGROUND ART

Current electric appliances demonstrate a tendency to have a reducedweight and a smaller size, which leads to development of lithium-ionsecondary batteries having a high energy density. Further, lithium-ionsecondary batteries are desired to have improved battery characteristicsas they are applied to more various fields. Battery characteristics willbecome more and more important particularly when lithium-ion secondarybatteries are put in use for automobiles.

Patent Literature 1 discloses a secondary battery including a positiveelectrode, a negative electrode, and a non-aqueous electrolyte solution,wherein the positive electrode includes an electrode compound whichabsorbs and releases an electrode reactant at a potential of 4.5 V orhigher (potential versus lithium), and the non-aqueous electrolytesolution includes a silyl compound where one or two or moresilicon-oxygen-containing groups (SiR₃—O—: the three Rs are each any oneof a monovalent hydrocarbon group and a halogenated group thereof) arebonded with an atom other than silicon.

Patent Literature 2 discloses an electrolyte solution for a high voltagebattery, containing a lithium salt, a non-aqueous solvent, and acompound represented by the formula (I):

(wherein R₁, R₂, and R₃ are independently selected from the groupconsisting of substituted and unsubstituted C₁-C₂₀ alkyl groups,substituted and unsubstituted C₁-C₂₀ alkenyl groups, substituted andunsubstituted C₁-C₂₀ alkynyl groups, and substituted and unsubstitutedC₅-C₂₀ aryl groups; X is nitrogen or oxygen; and Y is selected from thegroup consisting of hydride groups, halo groups, hydroxy groups, thiogroups, alkyl groups, alkenyl groups, alkynyl groups, aryl groups,iminyl groups, alkoxy groups, alkenoxy groups, alkynoxy groups, aryloxygroups, carboxy groups, alkylcarbonyloxy groups, alkenylcarbonyloxygroups, alkynylcarbonyloxy groups, arylcarbonyloxy groups, alkylthiogroups, alkenylthio groups, alkynylthio groups, arylthio groups, cyanogroups, N-substituted amino groups, alkylcarbonylamino groups,N-substituted alkylcarbonylamino groups, alkenylcarbonylamino groups,N-substituted alkenyl carbonylamino groups, alkynylcarbonyl aminogroups, N-substituted alkynylcarbonylamino groups, arylcarbonylaminogroups, N-substituted arylcarbonylamino groups, boron-containing groups,aluminum-containing groups, silicon-containing groups,phosphorus-containing groups, and sulfur-containing groups), wherein theelectrolyte solution is characterized by electrochemical stability in ahigh voltage battery at voltages above about 4.2 V.

Patent Literature 3 discloses a non-aqueous electrolyte solution for alithium secondary battery, including 0.01 to 15% by mass of anorganosilicon compound represented by the following formula [1] and 0.1to 3 mol/L of a fluorine-containing alkali metal salt that is anelectrolyte, wherein the amount of a fluorinated organosilicon compoundgenerated by reaction of the organosilicon compound and thefluorine-containing alkali metal salt is 0.2% by mass or less,

wherein H is a metal atom, a phosphorus atom, a boron atom, or P═O; R₁is a C1-C11 alkyloxy group, a silyloxy group, or a C1-C11 alkylsilyloxygroup; n is the number of R₁ that is bonded to M, and is equal to theoxidation number of M minus 1 or the oxidation number of M minus 3; whenn is 2 or greater, the two or more of R₁ may be the same as or differentfrom each other; and R₂ to R₄ are each independently a C1-C11 alkylgroup, a C1-C11 alkenyl group, a C1-C11 alkyloxy group, or a C6-C11 arylgroup.

Patent Literature 4 discloses an electrolyte for a lithium secondarybattery, including:

a compound represented by the following formula 1:

A-[OSi(C_(m)H_(2m+1))₃]₃   [formula 1]

(wherein A is phosphorus (P) or boron (B); and m is an integer of 0 to6);

a lithium salt; and

a non-aqueous organic solvent.

Patent Literature 5 discloses a non-aqueous electrolyte solution for alithium secondary battery containing an electrolyte and a non-aqueoussolvent that contains a compound represented by the following formula[1]:

wherein M is a metal element, phosphorus, or boron; R¹ is a C1-C11alkyloxy group or a silyloxy group; when n is 2 or greater, the two ormore of R¹ may be the same as or different from each other; R², R³, andR⁴ are the same as or different from each other, and are each a C1-C11alkyl, alkenyl, aryl, or alkyloxy group; and n is the number of R¹ thatis bonded to M, and is equal to the oxidation number of M minus 1.

CITATION LIST Patent Literature

-   Patent Literature 1: JP 2015-133278 A-   Patent Literature 2: JP 2014-522078 T-   Patent Literature 3: JP 5274563 B-   Patent Literature 4: JP 2008-130544 A-   Patent Literature 5: JP 2001-57237 A

SUMMARY OF INVENTION Technical Problem

Conventional electrolyte solutions cause an increase in resistance whenthey are used in electrochemical devices such as lithium ion secondarybatteries and the electrochemical devices are stored at hightemperature.

The invention is made in view of the above current state of the art, andaims to provide an electrolyte solution that can hardly increase theresistance of an electrochemical device such as a lithium ion secondarybattery even after the electrochemical device is stored at hightemperature in comparison with that before the storage.

The invention also aims to provide an electrochemical device such as alithium ion secondary battery that can hardly have an increasedresistance even after stored at high temperature in comparison with thatbefore the storage.

Solution to Problem

The inventors performed studies to find that selectingtris(trimethylsilyl) phosphite as an organosilicon compound andselecting a specific fluorinated cyclic carbonate among many solventsand additives to combine them can solve the above issues, therebycompleting the invention.

In other words, the invention relates to an electrolyte solutioncontaining tris(trimethylsilyl) phosphite and at least one fluorinatedsaturated cyclic carbonate (1) selected from the group consisting of acompound (1-a) and a compound (1-b),

-   the compound (1-a) being represented by the following formula:

-   the compound (1-b) being represented by the following formula:

The electrolyte solution preferably further contains a solvent.

The solvent preferably contains at least one selected from the groupconsisting of a non-fluorinated saturated cyclic carbonate, afluorinated saturated cyclic carbonate other than the fluorinatedsaturated cyclic carbonate (1), a non-fluorinated acyclic carbonate, afluorinated acyclic carbonate, a non-fluorinated acyclic ester, and afluorinated acyclic ester.

In the electrolyte solution, the tris(trimethylsilyl) phosphite ispreferably present in an amount of 0.001 to 10% by mass relative to thesolvent.

In the electrolyte solution, the fluorinated saturated cyclic carbonate(1) is preferably present in an amount of 0.001 to 10% by mass relativeto the solvent.

The electrolyte solution preferably further contains vinyl onecarbonate.

Preferably, the electrolyte solution further contains a solvent and thevinylene carbonate is present in an s amount of 0.001 to 10% by massrelative to the solvent,

The electrolyte solution preferably further contains an electrolytesalt.

The invention also relates to an electrochemical device including theabove electrolyte solution.

The invention also relates to a secondary battery including the aboveelectrolyte solution.

The invention also relates to a module including the aboveelectrochemical device or the above secondary battery,

Advantageous Effects of Invention

The electrolyte solution of the invention can reduce an increase inresistance even after the electrochemical device is stored at hightemperature.

The electrochemical device including the electrolyte solution of theinvention can hardly have an increased resistance even after stored athigh temperature.

DESCRIPTION OF EMBODIMENTS

The invention will be specifically described hereinbelow.

The electrolyte solution of the invention contains tris(trimethylsilyl)phosphite and at least one fluorinated saturated cyclic carbonate (1)selected from the group consisting of a compound (1-a) and a compound(1-b).

The tris(trimethylsilyl) phosphite is represented by the followingformula.

The compound (1-a) is represented by the following formula.

The formula (1-b) is represented by the following formula.

The electrolyte solution of the invention preferably contains a solvent.

When the electrolyte solution of the invention contains a solvent, thetris (trimethylsilyl) phosphite is preferably present in an amount of0.001 to 10% by mass relative to the solvent. The amount is morepreferably 0.01% by mass or more, still more preferably 0.1% by mass ormore, while more are preferably 5% by mass or leas, still morepreferably 1% by mass or less.

When the electrolyte solution of the invention contains a solvent, thefluorinated saturated cyclic carbonate {1} is preferably present in anamount of 0.001 to 10% by mass relative to the solvent. The amount ismore preferably 0.01% by mass or more, still more preferably 0.1% bymass or more, while more preferably 5% by mass or less, still morepreferably 1% by mass or less.

The solvent in the electrolyte solution is preferably present in anamount of 90% by volume or more, more preferably 95% by volume or mors,while may be 99.9% by volume or less, relative to the electrolytesolution.

The solvent preferably contains at least one selected from the groupconsisting of a non-fluorinated saturated cyclic carbonate, afluorinated saturated cyclic carbonate other than the fluorinatedsaturated cyclic carbonate (1), a non-fluorinated acyclic carbonate, afluorinated acyclic carbonate, a non-fluorinated acyclic ester, and afluorinated acyclic ester.

The solvent also preferably contains at least one cyclic carbonateselected from the group consisting of the non-fluorinated saturatedcyclic carbonate and the fluorinated saturated cyclic carbonate otherthan the fluorinated saturated cyclic carbonate (1) and at least oneacyclic compound selected from the group consisting of thenon-fluorinated acyclic carbonate, the fluorinated acyclic carbonate,the non-fluorinated acyclic ester, and the fluorinated acyclic ester.

The cyclic carbonate and the acyclic compound in the solvent preferablyhave a volume ratio of 10/90 to 90/10, more preferably 20/80 or higher,still more preferably 25/75 or higher, while more preferably 70/30 orlower, still more preferably 50/50 or lower.

The solvent also preferably contains the non-fluorinated saturatedcyclic carbonate and at least one non-fluorinated acyclic compoundselected from the group consisting of the non-fluorinated acycliccarbonate and the non-fluorinated acyclic ester. This combination issuitable when the electrolyte solution is used for an electrochemicaldevice that is driven at relatively low voltage.

The non-fluorinated saturated cyclic carbonate and the non-fluorinatedacyclic compound in the solvent preferably have a volume ratio of 10/90to 90/10, more preferably 20/80 or higher, still more preferably 25/75or higher, while more preferably 70/30 or lower, still more preferably50/50 or lower.

The solvent also preferably contains the fluorinated saturated cycliccarbonate other than the fluorinated saturated cyclic carbonate (1) andat least one fluorinated acyclic compound selected from the groupconsisting of the fluorinated acyclic carbonate and the fluorinatedacyclic ester. This combination is suitable when the electrolytesolution is used for an electrochemical device that is driven atrelatively high voltage.

The fluorinated saturated cyclic carbonate other than the fluorinatedsaturated cyclic carbonate (1) and the fluorinated acyclic compound inthe solvent preferably have a volume ratio of 10/90 to 90/10, morepreferably 20/80 or higher, still more preferably 25/75 or higher, whilemore preferably 70/30 or lower, still more preferably 50/50 or lower.

Examples of the non-fluorinated saturated cyclic carbonate includeethylene carbonate (EC), propylene carbonate (PC), and butylenecarbonate.

In order to give a high permittivity and good viscosity, thenon-fluorinated saturated cyclic carbonate preferably includes at leastone compound selected from the group consisting of ethylene carbonate,propylene carbonate, and butylene carbonate.

For the non-fluorinated saturated cyclic carbonate, one of the abovecompounds may be used or two or more thereof may be used in combination.

The fluorinated saturated cyclic carbonate other than the fluorinatedsaturated cyclic carbonate (1) is a saturated cyclic carbonate with afluorine atom added thereto. Specific examples thereof include acompound other than the fluorinated saturated cyclic carbonate (1)represented by the following formula (A):

wherein X³ to X⁴ are the same as or different from each other, and areeach —H, —CH₃, —C₂H₅, —F, a fluorinated alkyl group optionallycontaining an ether bond, or a fluorinated alkoxy group optionallycontaining an ether bond; at least one selected from X¹ to X⁴ is —F, afluorinated alkyl group optionally containing an ether bond, or afluorinated alkoxy group optionally containing an ether bond.

The presence of the fluorinated saturated cyclic carbonate in theelectrolyte solution of the invention when applied to a lithium-ionsecondary battery, for example, enables formation of a stable film onthe negative electrode and can thereby sufficiently reduce a sidereaction of the electrolyte solution on the negative electrode. Thisresults in significantly stable and excellent charge and dischargecharacteristics.

The term “ether bond” herein means a bond represented by —O—.

In order to give a good permittivity and oxidation resistance, one ortwo of X¹ to X⁴ is/are each preferably —F, a fluorinated alkyl groupoptionally containing an ether bond, or a fluorinated alkoxy groupoptionally containing an ether bond.

In anticipation of a decrease in viscosity at low temperature, anincrease in flash point, and improvement in solubility of an electrolytesalt, X¹ to X⁴ are each preferably —H, —F, a fluorinated alkyl group(a), a fluorinated alkyl group (b) containing an ether bond, or afluorinated alkoxy group (c).

The fluorinated alkyl group (a) is an alkyl group in which at least onehydrogen atom thereof is replaced by a fluorine atom. The fluorinatedalkyl group (a) preferably has a carbon number of 1 to 20, morepreferably 1 to 17, still more preferably 1 to 7, particularlypreferably 1 to 5.

Too large a carbon number may cause poor low-temperature characteristicsand low solubility of an electrolyte salt. Too small, a carbon numbermay cause low solubility of an electrolyte salt, low dischargeefficiency, and increased viscosity, for example.

Examples of the fluorinated alkyl group (a) having a carbon number of 1include CFH₂—, CF₂H—, and CF₃—. In order to give good high-temperaturestorage characteristics, CF₂H— or CF₃— is particularly preferred.

In order to give good solubility of an electrolyte salt, preferredexamples of the fluorinated alkyl group (a) having a carbon number of 2or greater include fluorinated alkyl groups represented by the followingformula (a-1):

R¹—R²—  (a-1)

wherein R¹ is an alkyl group having a carbon number of 1 or greater andoptionally containing a fluorine atom; R² is a C1-C3 alkylene groupoptionally containing a fluorine atom; and at least one selected from R¹and R² contains a fluorine atom.

R¹ and R² each may further contain an atom other than carbon, hydrogen,and fluorine atoms.

R¹ is an alkyl group having a carbon number of 1 or greater andoptionally containing a fluorine atom, is preferably a C1-C16 linear orbranched alkyl group. The carbon number of R¹ is more preferably 1 to 6,still more preferably 1 to 3.

Specifically, for example, CH₃—, CH₃CH₂—, CH₃CH₂CH₂—, CH₃CH₂CH₂CH₂—, andgroups represented by the following formulae:

may be mentioned as linear or branched alkyl groups for R¹,

Examples of R¹ which is a linear alkyl group containing a fluorine atominclude CF₃—, CF₃CH₂—, CF₃CF₂—, CF₃CH₂CH₂—, CF₃CF₂CH₂—, CF₃CF₂CF₂—,CF₃CH₂CF₂—, CF₃CH₂CH₂CH₂—, CF₃CF₂CH₂CH₂—, CF₃CH₂CF₂CH₂—, CF₃CF₂CF₂CH₂—,CF₃CF₂CF₂CF₂—, CF₃CF₂CH₂CF₂—, CF₃CH₂CH₂CH₂CH₂—, CF₃CF₂CH₂CH₂CH₂—,CF₃CH₂CF₂CH₂CH₂—, CF₃CF₂CF₂CH₂CH₂—, CF₃CF₂CF₂CF₂CH₂—, CF₃CF₂CH₂CF₂CH₂—,CF₃CF₂CH₂CH₂CH₂CH₂—, CF₃CF₂CF₂CF₂CH₂CH₂—, CF₃CF₂CH₂CF₂CH₂CH₂—, HCF₂—,HCF₂CH₂—, HCF₂CF₂—, HCF₂CH₂CH₂—, HCF₂CF₂CH₂—, HCF₂CH₂CF₂—,HCF₂CF₂CH₂CH₂—, HCF₂CH₂CF₂CH₂—, HCF₂CF₂CF₂CF₂—, HCF₂CF₂CH₂CH₂—,HCF₂CH₂CF₂CH₂CH₂—, HCF₂CF₂CF₂CF₂CH₂—, HCF₂CF₂CF₂CF₂CH₂CH₂—, FCH₂—,FCH₂CH₂—, FCH₂CF₂—, FCH₂CF₂CH₂—, FCH₂CF₂CF₂—, CH₃CF₂CH₂—, CH₃CF₂CF₂—,CH₃CF₂CH₂CF₂—, CH₃CF₂CF₂CF₂—, CH₃CH₂CF₂CF₂—, CH₃CF₂CH₂CF₂CH₂—,CH₃CF₂CF₂CF₂CH₂—, CH₃CF₂CF₂CH₂CH₂—, CH₃CH₂CF₂CF₂CH₂—,CH₃CF₂CH₂CF₂CH₂CH₂—, CH₃CF₂CH₂CF₂CH₂CH₂—, HCFClCF₂CH₂—, HCF₂CFClCH₂—,HCF₂CFClCF₂CFCICH₂—, and HCFClCF₂CFClCF₂CH₂—.

Examples of R¹ which is a branched alkyl group containing a fluorineatom include those represented by the following formulae.

The presence of a branch such as CH₃— or CF₃— may easily cause highviscosity. Thus, the number of such branches is more preferably small(one) or zero.

R² is a C1-C3 alkylene group optionally containing a fluorine atom. R²may be either linear or branched. Examples of a minimum structural unit,constituting such a linear or branched alkylene group are shown below.R² is constituted by one or combination of these units.

-   (i) Linear minimum structural units-   —CH₂—, —CHF—, —CF₂—, —CHCl—, —CFCl—, —CCl₂—-   (ii) Branched minimum structural units

Preferred among these exemplified units are Cl-free structural unitsbecause such units may not be dehydrochlorinated by a base, and thus maybe more stable.

R² which is a linear group consists only of any of the above linearminimum structural units, and is preferably —CH₂—, —CH₂CH₂—, or CF₂—. Inorder to further improve the solubility of an electrolyte salt, —CH₂— or—CH₂CH₂— is more preferred.

R² which is a branched group includes at least one of the above branchedminimum structural units. A preferred example thereof is a grouprepresented by —(CX^(a)X_(b))— (wherein X^(a) is H, F, CH₃, or CF₃;X^(b) is C₃ or CF₃; when X^(b) is CH₃, X^(a) is H or CH₃). Such a groupcan much further improve the solubility of an electrolyte salt.

For example, CF₃CF₂—, HCF₂CF₂—H₂CFCF₂—, CH₃CF₂—, CF₃CHF—, CH₃CF₂—,CF₃CF₂CF₂—, HCF₂CF₂CF₂—, H₂CFCF₂CF₂—, CH₃CF₂CF₂—, and those representedby the following formulae:

may be mentioned as preferred examples of the fluorinated alkyl group(a),

The fluorinated alkyl group (b) containing an ether bond is an alkylgroup containing an ether bond in which at least one hydrogen atomthereof is replaced by a fluorine atom. The fluorinated alkyl group (b)containing an ether bond preferably has a carbon number of 2 to 17. Toolarge a carbon number may cause high viscosity of the fluorinatedsaturated cyclic carbonate. This may also cause the presence of manyfluorine-containing groups, resulting in poor solubility of anelectrolyte salt due to reduction in permittivity, and poor miscibilitywith other solvents. Accordingly, the carbon number of the fluorinatedalkyl group (b) containing an ether bond is preferably 2 to 10, morepreferably 2 to 7.

The alkylene group which constitutes the ether moiety of the fluorinatedalkyl group (b) containing an ether bond is a linear or branchedalkylene group. Examples of a minimum structural unit constituting sucha linear or branched alkylene group are shown below.

-   (i) Linear minimum structural units-   —CH₂—, —CHF—, —CF₂—, —CHCl—, —CFCl—, —CCl₂—-   (ii) Branched minimum structural units

The alkylene group may be constituted by one of these minimum structuralunits, or may be constituted by multiple linear units (i), by multiplebranched units (ii), or by a combination of a linear unit (i) and abranched unit (ii). Preferred examples will be mentioned in detaillater.

Preferred among these exemplified units are Cl-free structural, unitsbecause such units may not be dehydrochlorinated by a base, and thus maybe more stable.

A still more preferred example of the fluorinated alkyl group (b)containing an ether bond is a group represented by the following formula(b-1):

R³—(OR⁴)_(n1)—  (b-1)

wherein R³ is preferably a C1-C6 alkyl group optionally containing afluorine atom; R⁴ is preferably a C1-C4 alkylene group optionallycontaining a fluorine atom; n1 is an integer of 1 to 3; and at least oneselected from R³ and R⁴ contains a fluorine atom.

Examples of R³ and R⁴ include the following groups, and any appropriatecombination of these groups can provide the fluorinated alkyl group (b)containing an ether bond represented by the formula (b-1). Still, thegroups are not limited thereto.

(1) R³ is preferably an alkyl group represented by X^(c) ₃C—(R⁵)_(n2)—wherein three X^(c)s are the same as or different from each other, andare each H or F; R⁵ is a C1-C5 alkylene group optionally containing afluorine atom; and n2 is 0 or 1.

When n2 is 0, R³ may be CH₃—, CF₃—, HCF₂—, or H₂CF—, for example.

When n2 is 1, specific examples of R³ which is a linear group includeCF₃CH₂—, CF₃CF₂—, CF₃CH₂CH₂—, CF₃CF₂CH₂—, CF₃CF₂CF₂—, CF₃CH₂CF₂—,CF₃CH₂CH₂CH₂—, CF₃CF₂CH₂CH₂—, CF₃CH₂CF₂CH₂—, CF₃CF₂CF₂CH₂—,CF₃CF₂CF₂CF₂—, CF₃CF₂CH₂CF₂—, CF₃CH₂CH₂CH₂CH₂—, CF₃CF₂CH₂CH₂CH₂—,CF₃CH₂CF₂CH₂CH₂—, CF₃CF₂CF₂CH₂CH₂—, CF₃CF₂CF₂CF₂CH₂—, CF₃CF₂CH₂CF₂CH₂—,CF₃CF₂CH₂CH₂CH₂CH₂—, CF₃CF₂CF₂CF₂CH₂CH₂—, CF₃CF₂CH₂CF₂CH₂CH₂—, HCF₂CH₂—,HCF₂CF₂—, HCF₂CH₂CH₂—, HCF₂CF₂CH₂—, HCF₂CH₂CF₂—, HCF₂CF₂CH₂CH₂—,HCF₂CH₂CF₂CH₂—, HCF₂CF₂CF₂CF₂—, HCF₂CF₂CH₂CH₂CH₂—, HCF₂CH₂CF₂CH₂CH₂—,HCF₂CF₂CF₂CF₂CH₂—, HCF₂CF₂CF₂CF₂CH₂CH₂—, FCH₂CH₂—, FCH₂CF₂—,FCH₂CF₂CH₂—, CH₃CF₂—, CH₃CH₂—, CH₃CF₂CH₂—, CH₃CF₂CF₂—, CH₃CH₂CH₂—,CH₃CF₂CH₂CF₂—, CH₃CF₂CF₂CF₂—, CH₃CH₂CF₂CF₂—, CH₃CH₂CH₂CH₂—,CH₃CF₂CH₂CF₂CH₂—, CH₃CF₂CF₂CF₂CH₂—, CH₃CF₂CF₂CH₂CH₂—, CH₃CH₂CF₂CF₂CH₂—,CH₃CF₂CH₂CF₂CH₂CH₂—, CH₃CH₂CF₂CF₂CH₂CH₃—, and CH₃CF₂CH₂CF₂CH₂CH₂—.

When n2 is 1, those represented by the following formulae:

may be mentioned as examples of R³ which is a branched group.

The presence of a branch such as CH₃— or CF₃— may easily cause highviscosity. Thus, R³ is more preferably a linear group.

(2) In —(OR⁴)_(n1)— of the formula (b-1), n1 is an integer of 1 to 3,preferably 1 or 2. When n1 is 2 or 3, R⁴s may be the same as ordifferent from each other.

Preferred specific examples of R⁴ include the following linear orbranched groups.

Examples of the linear groups include —CH₂—, —CHF—, —CF₂—, —CH₂CH₂—,—CF₂CH₂—, —CF₂CF₂—, —CH₂CF₂—, —CH₂CH₂CH₂—, —CH₂CH₂CF₂—, —CH₂CF₂CH₂—,—CH₂CF₂CF₂—, —CF₂CH₂CH₂—, —CF₂CF₂CH₂—, —CF₂CH₂CF₂, and —CF₂CF₂CF₂—.

Those represented by the following formulae:

may be mentioned as examples of the branched groups.

The fluorinated alkoxy group (c) is an alkoxy group in which at leastone hydrogen atom thereof is replaced by a fluorine atom. Thefluorinated alkoxy group (c) preferably has a carbon number of 1 to 17.The carbon number is more preferably 1 to 6.

The fluorinated alkoxy group (c) is particularly preferably afluorinated alkoxy group represented by X^(d) ₃C—(R⁶)_(n3)—O—, whereinthree X^(d)s are the same as or different from each other, and are eachH or F; R⁶ is preferably a C1-C5 alkylene group optionally containing afluorine atom; n3 is 0 or 1; and any of the three X^(d)s contain afluorine atom.

Specific examples of the fluorinated alkoxy group (c) includefluorinated alkoxy groups in which an oxygen atom binds to an end of analkyl group mentioned as an example for R¹ in the formula (a-1).

The fluorinated alkyl group (a), the fluorinated alkyl group (b)containing an ether bond, and the fluorinated alkoxy group (c) in thefluorinated saturated cyclic carbonate each preferably have a fluorinecontent, of 10% by mass or more. Too less a fluorine content may cause afailure in sufficiently achieving an effect of reducing the viscosity atlow temperature and an effect of increasing the flash point. Thus, thefluorine content is more preferably 12% by mass or more, still morepreferably 15% by mass or more. The upper limit thereof is usually 76%by mass.

The fluorine content of each of the fluorinated alkyl group (a), thefluorinated alkyl group (b) containing an ether bond, and thefluorinated alkoxy group (c) is a value calculated based on thecorresponding structural formula by the following formula:

{(Number of fluorine atoms×19)/(Formula weight of group)}×100(%).

In order to give good permittivity and oxidation resistance, thefluorine content, in the whole fluorinated saturated cyclic carbonate ispreferably 10% by mass or more, more preferably 15% by mass or more. Theupper limit thereof is usually 16% by mass.

The fluorine content in the fluorinated saturated cyclic carbonate is avalue calculated based on the structural formula of the fluorinatedsaturated cyclic carbonate by the following formula:

{(Number of fluorine atoms×19)/(Molecular weight of fluorinatedsaturated cyclic carbonate}×100(%).

Specific examples of the fluorinated saturated cyclic carbonate includethe following.

Specific examples of the fluorinated saturated cyclic carbonate in whichat least one selected, from X¹ to X⁴ is —F include those, represented bythe following formulae.

These compounds have a high withstand voltage and give good solubilityof an electrolyte salt.

Alternatively, those represented by the following formulae:

may also be used.

Those represented by the following formulae:

may be mentioned as specific examples of the fluorinated saturatedcyclic carbonate in which at least one selected from X¹ to X⁴ is afluorinated alkyl group (a) and the others are —H.

Those represented by the following formulae:

may be mentioned as specific examples of the fluorinated saturatedcyclic carbonate in which at least one selected from X¹ to X⁴ is afluorinated alkyl group (b) containing an ether bond or a fluorinatedalkoxy group (c) and the others are —H.

In particular, the fluorinated saturated cyclic carbonate is preferablyany of the following compounds.

The fluorinated saturated cyclic carbonate more preferably includes atleast one selected from the groups consisting of fluoroethylenecarbonate, difluoroethylene carbonate, and trifluoromethylethylenecarbonate.

Examples of the non-fluorinated acyclic carbonate includehydrocarbon-based acyclic carbonates such as CR₃OCOOCH₃ (dimethylcarbonate, DMC), CH₃CH₂OCOOCH₂CH₃ (diethyl carbonate, DEC),CH₃CH₂OCOOCH₃ (ethyl methyl carbonate, SMC), CH₃OCOOCH₂CH₂CH₃ (methylpropyl carbonate), methyl butyl carbonate, ethyl propyl carbonate, andethyl butyl carbonate. Preferred is at least one selected from the groupconsisting of ethyl methyl carbonate, diethyl carbonate, and dimethylcarbonate.

An example of the fluorinated acyclic carbonate is a compoundrepresented by the following formula (B):

Rf²OCOOR⁶   (B)

wherein Rf² is a C1-C7 fluorinated alkyl group; and R⁶ is a C1-C7 alkylgroup optionally containing a fluorine atom.

In order to be favorably used also at high voltage, the electrolytesolution of the invention preferably contains the fluorinated acycliccarbonate.

Rf² is a C1-C7 fluorinated alkyl group and is a C1-C7 alkyl groupoptionally containing a fluorine atom.

The fluorinated alkyl group is an alkyl group in which at least onehydrogen atom thereof is replaced by a fluorine atom. When R⁶ is analkyl group containing a fluorine atom, it is a fluorinated alkyl group.

In order to give low viscosity, Rf² and R⁶ each preferably have a carbonnumber of 2 to 7, more preferably 2 to 4.

Too large a carbon number may cause poor low-temperature characteristicsand low solubility of an electrolyte salt. Too small a carbon number maycause low solubility of an electrolyte salt, low discharge efficiency,and increased viscosity, for example.

Examples of the fluorinated alkyl group having a carbon number of 1include CFH₂—, CF₂H—, and CF₃—. In order to give high-temperaturestorage characteristics, particularly preferred is CF₂H— or CF₃—.

In order to give good solubility of an electrolyte salt, preferredexamples of the fluorinated alkyl group having a carbon number of 2 orgreater include fluorinated alkyl groups represented by the followingformula (d-1):

R¹—R²—  (d-1)

wherein is an alkyl group having a carbon number of 1 or greater andoptionally containing a fluorine atom; R² is a C1-C3 alkylene groupoptionally containing a fluorine atom; and at least one selected from R¹and R² contains a fluorine atom.

R¹ and R² each may further contain an atom other than carbon, hydrogen,and fluorine atoms.

R¹ is an alkyl group having a carbon number of 1 or greater andoptionally containing a fluorine atom. R¹ is preferably a C1-C6 linearor branched alkyl group. The carbon number of R¹ is more preferably 1 to6, still more preferably 1 to 3.

Specifically, for example, CH₃—, CH₃CH₂—, CH₃CH₂CH₂—, CH₃CH₂CH₂CH₂—, andthose represented by the following formulae:

may be mentioned as linear or branched alkyl groups for R¹.

Examples of R¹ which is a linear alkyl group containing a fluorine atominclude CF₃—, CF₃CH₂—, CF₃CF₂—, CF₃CH₂CH₂—, CF₃CF₂CH₂—, CF₃CF₂CF₂—,CF₃CH₂CF₂—, CF₃CH₂CH₂CH₂—, CF₃CF₂CH₂CH₂—, CF₃CH₂CF₂CH₂—, CF₃CF₂CF₂CH₂—,CF₃CF₂CF₂CF₂—, CF₃CF₂CH₂CF₂—, CF₃CH₂CH₂CH₂CH₂—, CF₃CF₂CH₂CH₂CH₂—,CF₃CH₂CF₂CH₂CH₂—, CF₃CF₂CF₂CH₂CH₂—, CF₃CF₂CF₂CF₂CH₂—, CF₃CF₂CH₂CF₂CH₂—,CF₃CF₂CH₂CH₂CH₂CH₂—, CF₃CF₂CF₂CF₂CH₂CH₂—, CF₃CF₂CH₂CF₂CH₂CH₂—, HCF₂—,HCF₂CH₂—, HCF₂CF₂—, HCF₂CH₂CH₂—, HCF₂CF₂CH₃—, HCF₂CH₂CF₂—,HCF₂CF₂CH₂CH₂—, HCF₂CH₂CF₂CH₂—, HCF₂CF₂CF₂CF₂—, HCF₂CF₂CH₂CH₂CH₂—,HCF₂CH₂CF₂CH₂CH₂—, HCF₂CF₂CF₂CF₂CH₂—, HCF₂CF₂CF₂CF₂CH₃CH₂—, FCH₂—,FCH₂CH₂—, FCH₂CF₂—, FCH₂CF₂CH₂—, FCH₂CF₂CF₂—, CH₃CF₂CH₂—, CH₃CF₂CF₂—,CH₃CF₂CH₂CF₂—, CH₃CF₂CF₂CF₂—, CH₃CH₂CF₂CF₂—, CH₃CF₂CH₃CF₂CH₂—,CH₃CF₂CF₂CF₂CH₂—, CH₃CF₂CF₂CH₂CH₂—, CH₃CH₂CF₂CF₂CH₂—,CH₃CF₂CH₂CF₂CH₂CH₂—, CH₃CF₂CH₂CF₂CH₂CH₂—, HCFClCF₂CH₂—, HCF₂CFClCH₂—,HCF₂CFClCF₂CFClCH₂—, and HCFClCF₂CFClCF₂CH₂—.

Examples of R¹ which is a branched alkyl group containing a fluorineatom include those represented by the following formulae.

The presence of a branch such as CH₃— or CF₃— may easily cause highviscosity. Thus, the number of such branches is mere preferably small(one) or zero.

R² is a C1-C3 alkylene group optionally containing a fluorine atom maybe either linear or branched. Examples of a minimum structural unitconstituting such a linear or branched alkylene group are shown below,R² is constituted by one or combination of these units.

-   (i) Linear minimum structural units-   —CH₂—, —CHF—, —CF₂—, —CHCl—, —CFCl—, —CCl₂—-   (ii) Branched minimum structural units

Preferred among these exemplified units are Cl-free structural unitsbecause such units may not be dehydrochlorinated by a base, and thus maybe more stable.

R² which is a linear group consists only of any of the above linearminimum structural units, and is preferably —CH₂—, —CH₂CH₂—, or —CF₂—.In order to further i improve the solubility of an electrolyte salt,—CH₂— or —CH₂CH₂— is more preferred.

R² which is a branched group includes at least one of the above branchedminimum structural units. A preferred example thereof is a grouprepresented by —(CX^(a)X^(b))—

(wherein X^(a) is H, F, CH₃, or CF₃; X^(b) is CH₃ or CF₃; when X^(b) isCF₃, X^(a) is H or CH₃). Such a group can much further improve thesolubility of an electrolyte salt.

For example, CF₃CF₂—, HCF₂CF₂—, H₂CFCF₂—, CH₃CF₂—, CF₃CH₂—, CF₃CF₂CF₂—,HCF₂CF₂CF₂—, H₂CFCF₂CF₂—, CH₃CF₂CF₂—, and those represented by thefollowing formulae;

may be mentioned as preferred examples of the fluorinated alkyl group.

The fluorinated alkyl group for Rf² and R⁶ is preferably CF₃—, CF₃CF₂—,(CF₃)₂CH—, CF₃CH₂—, C₂F₅CH₂—, CF₃CF₂CH₂—, HCF₂CF₂CH₂—, orCF₃CF₃CFHCF₂CH₂—. In order to give high incombustibility and good ratecharacteristics and oxidation resistance, more preferred are CF₃CH₂—,CF₃CF₂CH₂—, and HCF₂CF₂CH₂—.

R⁶, when it is an alkyl group free from a fluorine atom, is a C1-C7alkyl group. In order to give low viscosity, R⁶ preferably has a carbonnumber of 1 to 4, more preferably 1 to 3.

Examples of the alkyl group free from a fluorine atom include CH₃—,CH₃CH₂—, (CH₃)₂CH—, and C₃H₇—. In order to give low viscosity and goodrate characteristics, preferred are CH₃— and CH₃CH₂—.

The fluorinated acyclic carbonate preferably has a fluorine content of20 to 70% by mass. The fluorinated acyclic carbonate having a fluorinecontent within the above range can maintain the miscibility with asolvent and the solubility of a salt. The fluorine content is morepreferably 30% by mass or more, still more preferably 35% by mass ormore, while more preferably 60% by mass or less, still more preferably50% by mass or less.

In the invention, the fluorine content is a value calculated based onthe structural formula of the fluorinated acyclic carbonate by thefollowing formula:

{(Number of fluorine atoms×19)/Molecular weight of fluorinated acycliccarbonate}×100(%).

In order to give low viscosity, the fluorinated acyclic carbonate ispreferably any of the following compounds.

Examples of the non-fluorinated acyclic ester include those containing 3to 7 carbon atoms in total in the structural formula thereof. Specificexamples thereof include methyl acetate, ethyl acetate, n-propylacetate, isopropyl acetate, n-butyl acetate, isobutyl acetate, t-butylacetate, methyl propionate, ethyl propionate, n-propyl propionate,isopropyl propionate, n-butyl propionate, isobutyl propionate, t-butylpropionate, methyl butyrate, ethyl butyrate, n-propyl butyrate,isopropyl butyrate, methyl isobutyrate, ethyl isobutyrate, n-propylisobutyrate, and isopropyl isobutyrate.

In order to improve the ion conductivity owing to reduction inviscosity, preferred among these are methyl acetate, ethyl acetate,n-propyl acetate, n-butyl acetate, methyl propionate, ethyl propionate,n-propyl propionate, isopropyl propionate, methyl butyrate, and ethylbutyrate.

In order to achieve good miscibility with other solvents and to givegood oxidation resistance, the fluorinated acyclic ester is preferably afluorinated acyclic ester represented by the following formula (3):

Rf³¹COOR³²

wherein Rf³¹ is a C1-C4 fluorinated alkyl group; and Rf³² is a C1-C4alkyl group optionally containing a fluorine atom.

Examples of Rf³¹ include HCF₂—, CF₃—, CF₃CF₂—, HCF₂CF₂—, CH₃CF₂—, andCF₃CH₂—. In order to achieve good viscosity and oxidation resistance,particularly preferred are HCF₂—, CF₃—, CF₃CF₂—, and CF₃CH₂−.

Examples of Rf³² include CH₃—, C₂H₅—, CF₃—, CF₃CF₂—, (CF₃)₂CH—, CF₃CH₂—,CF₃CH₂CH₂—, CF₃CFHCF₂CH₂—, C₂F₅CH₂—, CF₂HCF₂CH₂—, C₂F₅CH₂CH₂—,CF₃CF₂CH₂—, and CF₃CF₂CF₂CH₂—. In order to achieve good miscibility withother solvents, particularly preferred are CH₃—, C₂H₅—, CF₃CH₂—, andCF₃CH₂CH₂—.

Specific examples of the fluorinated acyclic ester include one or two ormore of CF₃CH₂C(═O)OCH₃, HCF₂C(═O)OCH₃, CF₃C(═O)OCH₂CH₂CF₃,CF₃C(═O)OCH₂C₂F₅, CF₃C(═O)OCH₂CF₂CF₂H, CF₃C(═O)OCH₂CF₃, andCF₃C(═O)OCH(CF₃)₂. In order to achieve good miscibility with othersolvents and to give rate characteristics, particularly preferred amongthese are CF₃CH₂C(=O)OCH₃, HCF₂C(═O)OCH₃, CF₃C(═O)OCH₂C₂F₅,CF₃C(═O)OCH₂CF₂CF₂H, CF₃C(═O)OCH₂CF₃, and CF₃C(═O)OCH(CF₃)₂.

In order to reduce an increase in resistance even after high-temperaturestorage as well as to give a lower resistance than that before thestorage, the electrolyte solution of the invention may preferablyfurther contain vinylene carbonate.

The vinylene carbonate is preferably present in an amount of 0.001 to10% by mass, more preferably 0.01% by mass or more, still morepreferably 0.1% by mass or more, while more preferably 5% by mass orless, still more preferably 1% by mass or less, relative to the solvent.

The electrolyte solution of the invention preferably contains anelectrolyte salt.

Examples of the electrolyte salt used include alkali metal salts,alkaline earth metal salts, metal salts with aluminum serving as acation, and ammonium salts, as well as any of those to be used for anelectrolyte solution such as liquid salts (ionic liquids), inorganicpolymer salts, and organic polymer salts.

The electrolyte salt of the electrolyte solution for a lithium-ionsecondary battery is preferably an alkali metal salt, more preferably alithium salt.

Any lithium salt may be used. Specific examples thereof include thefollowing:

inorganic lithium salts such as LiPF₆, LiBF₄, LiClO₄, LiAlF₄, LiSbF₆,LiTaF₆, and LiWF₇;

lithium fluorophosphates such as LiPO₃F and LiPO₂F₂;

lithium tungstates such as LiWOF₅;

lithium carboxylates such as HCO₂Li, CH₃CO₂Li, CH₂FCO₂Li, CHF₂CO₂Li,CF₃CO₂Li, CF₃CH₂CO₂Li, CF₃CF₂CO₂Li, CF₃CF₂CF₂CO₂Li, andCF₃CF₂CF₂CF₂CO₂Li;

lithium sulfonates such as FSO₃Li, CH₃SO₃Li, CH₂FSO₃Li, CHF₂SO₃Li,CF₃SO₃Li, CF₃CF₂SO₃Li, CF₃CF₂CF₂SO₃Li, and CF₃CF₂CF₂CF₂SO₃Li;

lithium imide salts such as LiN(FCO)₂, LiN(FCO) (FSO₂), LiN(FSO₂)₂,LiN(FSO₂)(CF₃SO₂), LiN(CF₃SO₂)₂, LiN(C₂F₅SO₂)₂, lithium cyclic1,2-perfluoroethanedisulfonylimide, lithium cyclic1,3-perfluoropropanedisulfonylimide, and LiN(CF₃SO₂)(C₄F₉SO₂);

lithium methide salts such as LiC(FSO₂)₃, LiC(CF₃SO₂)₃, andLiC(C₂F₆SO₂)₃;

lithium oxalatoborates such as lithium difluorooxalatoborate and lithiumbis (oxalate)borate;

lithium oxalatophosphates such as lithium tetrafluorooxalatophosphate,lithium difluorobis(oxalate)phosphate, and lithiumtris(oxalate)phosphate; and

fluorine-containing organic lithium salts such as salts represented bythe formula: LiPF_(a)(C_(n)F_(2n+1))_(6-a) (wherein a is an integer of 0to 5; and n is an integer of 1 to 6) (e.g., LiPF₄(CF₃)₂, LiPF₄(C₂F₅)₂),LiPF₄(CF₃SO₂)₂, LiPF₄(C₂F₅SO₂)₂, LiBF₃CF₃, LiBF₃C₂F₅, LiBF₃C₃F₇,LiBF₂(CF₃)₂, LiBF₂(C₂F₅)₂, LiBF₂(CF₃SO₂)₂, and LiBF₂(C₂F₅SO₂)₂.

Preferred among these salts is at least one selected from the groupconsisting of LiPF₆, LiBF₄, LiSbF₆, LiTaF₆, LiPO₂F₂, FSO₃Li, CF₃SO₃Li,LiN(FSO₂)₂, LiN(FSO₂)(CF₃SO₂), LiN(CF₃SO₂)₂, LiN(C₂F₅SO₂)₂, lithiumcyclic 1,2-perfluoroethanedisulfonylimide, lithium cyclic1,3-perfluoropropanedisulfonylimde, LiC(FSO₂)₃, LiC(CF₃SO₂)₃,LiC(C₂FS₅O₂)₃, lithium bis(oxalate)borate, lithiumdifluorooxalatoborate, lithium tetrafluorooxalatophosphate, lithiumdifluorobis (oxalate)phosphate, LiBF₃CF₃, LiBF₃C₂F₅, and saltsrepresented by the formula: LiPF_(a)(C_(n)F_(2n+1))_(6-a) (wherein a isan integer of 0 to 5; and n is an integer of 1 to 6).

Examples of the salts represented by the formula:LiPF_(a)(C_(n)F_(2n+1))_(6-a) include LiPF₃(CF₃)₃, LiPF₃(C₂F₅)₃,LiPF₃(C₃F₇)₃, LiPF₃(C₄F₉)₃, LiPF₄(CF₃)₂, LiPF₄(C₂F₅)₂, LiPF₄(C₃F₇)₂, andLiPF₄(C₄F₉)₂ (each alkyl group represented by C₃F₇ or C₄F₉ in theformulae may have either a linear structure or a branched structure).

These lithium salts may be used alone or in combination of two or more,in combination use of two or more thereof, preferred examples thereofinclude a combination of LiPF₆ and LiBF₄, a combination of LiPF₆ andFSO₃Li, and a combination of LiPF₆ and LiPO₂F₂, each of which have aneffect of improving the load characteristics and the cyclecharacteristics.

Preferred among these are a combination use of LiPF₆ and FSO₃Li and ofLiPF₆ and LiPO₂F₂ because the effects thereof are significant.Particularly preferred among these is a combination use of LiPF₅ andLiPO₂F₂ because it can lead to significant effects even when added inslight amounts.

In the case of a combination use of LiPF₆ and LiBF₄ or of LiPF₆ andFSO₃Li, LiBF₄ or FSO₃Li may be used in any amount that does notsignificantly impair the effects of the invention in 100% by mass of thewhole electrolyte solution. The amount thereof is usually 0.01% by massor more, preferably 0.1% by mass or more, while the upper limit thereofis usually 30% by mass or less, preferably 20 % by mass or less,relative to the electrolyte solution.

In another example, an inorganic lithium salt and an organic lithiumsalt are used in combination. Such a combination has an effect ofreducing deterioration due to high-temperature storage.

The organic lithium salt is preferably CF₃SO₃Li, LiN(FSO₂)₂,LiN(FSO₂)(CF₃SO₂), LiN(CF₃SO₂)₂, LiN(C₂F₅SO₂)₂, lithium cyclic1,2-perfluoroethanedisulfonylimide, lithium cyclic1,3-perfluoropropanedisulfonylimide, LiC(FSO₂)₃, LiC(CF₃SO₂)₃,LiC(C₂F₅SO₂)₃, lithium bis(oxalate)borate, lithiumdifluorooxalatoborate, lithium tetrafluorooxalatophosphate, lithiumdifluorobis(oxalato)phosphate, LiBF₃CF₃, LiBF₃C₂F₅, LiPF₃(CF₃)₃,LiPF₃(C₂F₅)₃, or the like.

In this case, the proportion of the organic lithium salt is preferably0.1% by mass or more, particularly preferably 0.5% by mass or more,while preferably 30% by mass or less, particularly preferably 20% bymass or less, in 100% by mass of the whole electrolyte solution.

The lithium salt in the electrolyte solution may have any concentrationthat does not impair the effects of the invention.

In order to make the electric conductivity of the electrolyte solutionwithin a favorable range and to ensure good battery performance, thelithium in the electrolyte solution preferably has a total moleconcentration of 0.3 mol/L or higher, more preferably 0.4 mol/L orhigher, still more preferably 0.5 mol/L or higher, while preferably 3mol/L or lower, more preferably 2.5 mol/L or lower, still morepreferably 2.0 mol/L or lower.

Too low a total mole concentration of lithium may cause insufficientelectric conductivity of the electrolyte solution, while too high aconcentration may cause an increase in viscosity and then reduction inelectric conductivity, impairing the battery performance.

The electrolyte salt in the electrolyte solution for an electric doublelayer capacitor is preferably an ammonium salt.

Examples of the ammonium salt include the following salts (IIa) to(IIe).

(IIa) Tetraalkyl Quaternary Ammonium Salts

Preferred examples thereof include tetraalkyl quaternary ammonium saltsrepresented by the following formula (IIa):

(wherein R^(1a), R^(2a), R^(3a), and R^(4a) are the same as or differentfrom each other, and are each a C1-C6 alkyl group optionally containingan ether bond; and X⁻ is an anion). In order to improve the oxidationresistance, any or all of the hydrogen atoms in the ammonium salt arealso preferably replaced by a fluorine atom and/or a C1-C4 fluorinatedalkyl group.

Preferred specific examples thereof include tetraalkyl quaternaryammonium salts represented by the following formula (IIa-1):

[Chem. 40]

(R^(1a))_(x)(R^(2a))_(y)N^(⊕)X⁶³   (IIa-1)

(wherein R^(1a), R^(2a), and X⁻ are defined as described above; x and yare the same as or different from each other, and are each art integerof 0 to 4 with x+y=4), and

alkyl ether group-containing trialkyl ammonium salts represented by thefollowing formula (IIa-2);

(wherein R^(5a) is a C1-C6 alkyl group; R^(6a) is a C1-C6 divalenthydrocarbon group; R^(7a) is a C1-C4 alkyl group; z is 1 or 2; and X⁻ isart anion). Introduction of an alkyl ether group enables reduction inviscosity.

The anion X⁻ may be either an inorganic anion or an organic anion.Examples of the inorganic anion include AlCl₄ ⁻, BF₄ ⁻, PF₆ ⁻, AsF₆ ⁻,TaF₆ ⁻, I⁻, and SbF₆ ⁻. Examples of the organic anion include CF₃COO⁻,CF₃SO₃ ⁻, (CF₃SO₂)₂N⁻, and (C₂F₅SO₂)₂N⁻.

In order to achieve good oxidation resistance and ionic dissociation,BF₄ ⁻, PF₆ ⁻, AgF₆ ⁻, and SbF₆ ⁻ are preferred.

Preferred specific examples of the tetraalkyl quaternary ammonium saltsto be used include Et₄NBF₄, Et₄NClO₄, Et₄NPF₆, Et₄NAsF₆, Et₄NSbF₆,Et₄NCF₃SO₃, Et₄N(CF₃SO₂)₂N, Et₄NC₄F₉SO₃, Et₃MeNBF₄, Et₃MeMClO₄,Et₃MeNPF₆, Et₃MeNAsF₆, Et₃MeNSbF₆, Et₃MeNCF₃SO₃, Et₃MeN(CF₃SO₂)₂N,Et₃MeNC₄F₉SO₃, and N,N-diethyl-N-methyl-N-(2-methoxyethyl)ammoniumsalts. In particular, Et₄NBF₄, Et₄NPF₆, Et₄SbF₆, Et₄NAsF₆, Et₃MeNBF₄,and an N,N-diethyl-N-methyl-N-(2-methoxyethyl)ammonium salt may bementioned as examples.

(IIb) Spirocyclic Bipyrrolidinium Salts

Preferred examples thereof include

spirocyclic bipyrrolidinium salts represented by the following formula(IIb-1):

(wherein R^(8a) and R^(9a) are the same as or different from each other,and are each a C1-C4 alkyl group; X⁻ is an anion; n1 is an integer of 0to 5 and n2 is an integer of 0 to 5),

spirocyclic bipyrrolidinium salts represented by the following formula(IIb-2):

(wherein R^(10a) and R^(11a) are the same as or different from eachother, and are each a C1-C4 alkyl group X⁻ is an anion n3 is an integerof 0 to 5; and n4 is an integer of 0 to 5), and

spirocyclic bipyrrolidinium salts represented by the following formula(IIb-3):

(wherein R^(12a) and R^(13a) are the same as or different from eachother, and are each a C1-C4 alkyl group; X⁻ is an anion; n5 is aninteger of 0 to 5; and n6 is an integer of 0 to 5). In order to improvethe oxidation resistance, any or all of the hydrogen atoms in thespirocyclic bipyrrolidinium salt are also preferably replaced by afluorine atom and/or a C1-C4 fluorinated alkyl group.

Preferred specific examples of the anion X⁻ are the same as thosementioned for the salts (IIa). In order to achieve good dissociation anda low internal resistance under high voltage, BF₄ ⁻, PF₆ ⁻, (CF₃SO₂)₂N⁻,or (C₂F₅SO₂)₂N⁻ is particularly preferred.

For example, those represented by the following formulae:

may be mentioned as preferred specific examples of the spirocyclicbipyrrolidinium salts.

These spirocyclic bipyrrolidinium salts are excellent, in solubility ina solvent, oxidation resistance, and ion conductivity.

(IIc) Imidazolium Salts

Preferred examples thereof include imidazolium salts represented by thefollowing formula (IIc):

wherein R^(14a) and R^(15a) are the same as or different from eachother, and are each a C1-C6 alkyl group; and X⁻ is an anion.

In order to improve the oxidation resistance, any or all of the hydrogenatoms in the imidazolium salt are also preferably replaced by a fluorineatom and/or a C1-C4 fluorinated alkyl group.

Preferred specific examples of the anion X⁻ are the same as thosementioned for the salts (IIa).

For example, one represented by the following formula:

may be mentioned as a preferred specific example thereof.

This imidazolium salt is excellent in that it has low viscosity and goodsolubility.

(IId) N-alkylpyridinium Salts

Preferred examples thereof include N-alkylpyridinium salts representedby the following formula (IId);

wherein R^(16a) is a C1-C6 alkyl group; and X⁻is an anion.

In order to improve the oxidation resistance, any or all of the hydrogenatoms in the N-alkylpyridinium salt are also preferably replaced by afluorine atom and/or a C1-C4 fluorinated alkyl group.

Preferred specific examples of the anion X⁻ are the same as thosementioned for the salts (IIa).

For example, those represented by the following formulae:

may be mentioned as preferred specific examples thereof.

These N-alkylpyridinium salts are excellent in that they have lowviscosity and good solubility.

(IIe) N,N-dialkylpyrrolidinium Salts

Preferred examples thereof include N,N-dialkylpyrrolidinium saltsrepresented by the following formula (IIe);

wherein R^(17a) and R^(18a) are the same as or different from eachother, and are each a C1-C6 alkyl group; and X⁻ is an anion.

In order to improve the oxidation resistance, any or all of the hydrogenatoms in the N,N-dialkylpyrrolidinium salt are also preferably replacedby a fluorine atom and/or a C1-C4 fluorinated alkyl group,

Preferred specific examples of the anion X⁻ are the same as thosementioned for the salts (IIa).

For example, those represented by the following formulae:

may be mentioned as preferred specific examples thereof.

These N,N-dialkylpyrrolidinium salts are excellent in that they have lowviscosity and good solubility.

Preferred among these ammonium salts are those represented by theformula (IIa), (IIb), or (IIe) because they can have good solubility,oxidation resistance, and ion conductivity. More preferred are thoserepresented by the following formulae:

wherein Me is a methyl group; Et is an ethyl group; and X⁻, x, and y aredefined as in the formula (IIa-1).

A lithium salt may be used as an electrolyte salt for an electric doublelayer capacitor. Preferred examples thereof include LiPF₆, LiBF₄,LiAsF₆, LiSbF₆, and LiN(SO₂C₂B₅)₂.

In order to further increase the capacity, a magnesium salt may be used.Preferred examples of the magnesium salt include Mg(ClO₄)₂ andMg(OOC₂H₅)₂.

The ammonium salt serving as an electrolyte salt is preferably used at aconcentration of 0.6 mol/L or higher. The ammonium salt at aconcentration lower than 0.6 mol/L may cause not only poorlow-temperature characteristics but also high initial internalresistance. The concentration of the electrolyte salt is more preferably0.9 mol/L or higher.

In order to give good low-temperature characteristics, the concentrationis preferably 3.0 mol/L or lower, more preferably 2 mol/L or lower.

In order to give excellent low-temperature characteristics, theconcentration is preferably 0.3 to 1.9 mol/L when the ammonium salt istriethyl methyl ammonium tetrafluoroborate (TSMABF₄).

The concentration is preferably 0.7 to 2.0 mol/L when the ammonium saltis spirobipyrrolidinium tetrafluoroborate (SBPBF₄).

The electrolyte solution of the invention preferably contains apolyethylene oxide that has a weight average molecular weight of 2000 to4000 and has —OH, —OCOOH, or —COOH at an end.

The presence of such a compound can improve the stability at theinterfaces with the respective electrodes, improving the characteristicsof an electrochemical device.

Examples of the polyethylene oxide include polyethylene oxide monool,polyethylene oxide carboxylate, polyethylene oxide diol, polyethyleneoxide dicarbaxylate, polyethylene oxide triol, and polyethylene oxidetricarboxylate. These may be used alone or in combination of two ormore.

In order to give better characteristics of an electrochemical device,preferred are a mixture of polyethylene oxide monool and polyethyleneoxide diol and a mixture of polyethylene oxide carboxylate andpolyethylene oxide dicarboxylate.

The polyethylene oxide having too small a weight average molecularweight may be easily oxidatively decomposed. The weight averagemolecular weight is more preferably 3000 to 4000.

The weight average molecular weight can be determined by gel permeationchromatography (GPC) in polystyrene equivalent.

The polyethylene oxide is preferably present in an amount of 1×10⁻⁶ to1×10⁻² mol/kg in the electrolyte solution. Too large an amount of thepolyethylene oxide may cause poor characteristics of an electrochemicaldevice.

The amount of the polyethylene oxide is more preferably 5×10⁻⁶ mol/kg ormore.

The electrolyte solution of the invention may further contain any ofother components such as an unsaturated cyclic carbonate, an overchargeinhibitor, and a known different aid. This can reduce impairment of thecharacteristics of an electrochemical device.

The unsaturated cyclic carbonate is a cyclic carbonate containing anunsaturated bond, i.e., a cyclic carbonate that contains at least onecarbon-carbon unsaturated bond in the molecule. Examples of theunsaturated cyclic carbonate include vinylene carbonate derivativesother than vinylene carbonate, ethylene carbonate derivativessubstituted with a substituent that contains an aromatic ring, acarbon-carbon double bond, or a carbon-carbon triple bond, phenylcarbonate derivatives, vinyl carbonate derivatives, allyl carbonatederivatives, and catechol carbonate derivatives.

Examples of the vinylene carbonate derivatives other than vinylenecarbonate include methylvinylene carbonate, 4,5-dimethylvinylenecarbonate, 4,5-diethylvinylene carbonate, phenylvinylene carbonate,4,5-diphenylvinylene carbonate, vinyivinylene carbonate,4,5-divinylvinylene carbonate, allylvinylene carbonate,4,5-diallylvinylene carbonate, 4-fluorovinylene carbonate,4-fluoro-5-methylvinylene carbonate, 4-fluoro-5-phenylvinylenecarbonate, 4-fluoro-5-vinyivinylene carbonate, and4-allyl-5-fluorovinylene carbonate.

Specific examples of the ethylene carbonate derivatives substituted witha substituent that contains an aromatic ring, a carbon-carbon doublebond, or a carbon-carbon triple bond include vinylethylene carbonate,4,5-divinylethylene carbonate, 4-methyl-5-vinylethylene carbonate,4-allyl-5-vinylethylene carbonate, ethynylethylene carbonate,4,5-diethynylethylene carbonate, 4-methyl-5-ethynylethylene carbonate,4-vinyl-5-ethynylethylene carbonate, 4-allyl-5-ethynylethylenecarbonate, phenylethylene carbonate, 4,5-diphenylethylene carbonate,4-phenyl-5-vinylethylene carbonate, 4-allyl-5-phenylethylene carbonate,allylethylene carbonate, 4,5-diallylethylene carbonate, and4-methyl-5-allylethylene carbonate,.

The unsaturated cyclic carbonate is preferably methylvinylene carbonate,4,5-dimethylvinylene carbonate, vinylvinylene carbonate,4,5-vinylvinylene carbonate, allylvinylene carbonate,4,5-diallylvinylene carbonate, vinylethylene: carbonate,4,5-divinylethylene carbonate, 4-methyl-5-vinylethylene carbonate,allylethylene carbonate, 4,5-diallylethylene carbonate,4-methyl-5-allylethylene carbonate, 4-allyl-5-vinylethylene carbonate,ethynylethylene carbonate, 4,5-diethynylethylene carbonate,4-methyl-5-ethynylethylene carbonate, and 4-vinyl-5-ethynylethylenecarbonate. In order to form a more stable interface protecting film,particularly preferred are vinylethylene carbonate and ethynylethylenecarbonate.

The unsaturated cyclic carbonate may have any molecular weight that doesnot significantly impair the effects of the invention. The molecularweight is preferably 50 or higher and 250 or lower. The unsaturatedcyclic carbonate having a molecular weight within this range can easilyensure its solubility in the electrolyte solution and can easily lead tosufficient achievement of the effects of the invention. The molecularweight of the unsaturated cyclic carbonate is more preferably 80 orhigher, while more preferably 150 or lower.

The unsaturated cyclic carbonate may be produced by any productionmethod, and may be produced by a known method selected as appropriate.

The unsaturated cyclic carbonates may be used alone or in anycombination of two or more at any ratio.

The unsaturated cyclic carbonate may be contained in any amount thatdoes not significantly impair the effects of the invention. The amountof the unsaturated cyclic carbonate is preferably 0.001% by mass ormore, more preferably 0.01% by mass or more, still more preferably 0.1%by mass or more, of 100% by mass of the solvent in the invention. Theamount is preferably 5% by mass or less, more preferably 4% by mass orless, still more preferably 3% by mass or less. The unsaturated cycliccarbonate in an amount within the above range allows an electrochemicaldevice containing the electrolyte solution to easily exhibit asufficient effect of improving the cycle characteristics, and can easilyavoid a situation with impaired high-temperature storagecharacteristics, generation of a large amount of gas, and a reduceddischarge capacity retention.

In addition to the aforementioned non-fluorinated unsaturated cycliccarbonates, a fluorinated unsaturated cyclic carbonate may also suitablybe used, as an unsaturated cyclic carbonate. The fluorinated unsaturatedcyclic carbonate is a cyclic carbonate containing an unsaturated bondand a fluorine atom.

The number of fluorine atoms in the fluorinated unsaturated cycliccarbonate may be any number that is 1 or greater. The number of fluorineatoms is usually 6 or smaller, preferably 4 or smaller, most preferably1 or 2.

Examples of the fluorinated unsaturated cyclic carbonate includefluorinated vinylene carbonate derivatives, and fluorinated ethylenecarbonate derivatives substituted with a substituent that, contains anaromatic ring or a carbon-carbon double bond.

Examples of the fluorinated vinylene carbonate derivatives include4-fluorovinylene carbonate, 4-fluoro-5-methylvinylene carbonate,4-fluoro-5-phenylvinylene carbonate, 4-allyl-5-fluorovinylene carbonate,and 4-fluoro-5-vinyivinylene carbonate.

Examples of the fluorinated ethylene carbonate derivatives substitutedwith a substituent that contains an aromatic ring or a carbon-carbondouble bond include 4-fluoro-4-vinylethylene carbonate,4-fluoro-4-allylethylene carbonate, 4-fluoro-5-vinylethylene carbonate,4-fluoro-5-allylethylene carbonate, 4,4-difluoro-4-vinylethylenecarbonate, 4,4-difluoro-4-allylethylene carbonate,4,5-difluoro-4-vinylethylene carbonate, 4,5-difluoro-4-allylethylenecarbonate, 4-fluoro-4,5-divinyl ethylene carbonate,4-fluoro-4,5-diaylyethylene carbonate, 4,5-difluoro-4,5-divinylethylenecarbonate, 4,5-difluoro-4,5-diallylethylene carbonate,4-fluoro-4-phenylethylene carbonate, 4-fluoro-5-phenylethylenecarbonate, 4,4-difluoro-5-phenylethylene carbonate, and4,5-difluoro-4-phenylethylene carbonate.

In order to form a stable interface protecting film, more preferred asthe fluorinated unsaturated cyclic carbonate are 4-fluorovinylenecarbonate, 4-fluoro-5-methylvinylene carbonate, 4-fluoro-5-vinyivinylenecarbonate, 4-allyl-5-fluorovinylene carbonate, 4-fluoro-4-vinylethylenecarbonate, 4-fluoro-4-allylethylene carbonate, 4-fluoro-5-vinylethylenecarbonate, 4-fluoro-5-allylethylene carbonate,4,4-difluoro-4-vinylethylene carbonate, 4,4-difluoro-4-allylethylenecarbonate, 4,5-difluoro-4-vinylethylene carbonate,4,5-difluoro-4-allylethylene carbonate, 4-fluoro-4,5-divinylethylenecarbonate, 4-fluoro-4,5-diallylethylene carbonate,4,5-difluoro-4,5-divinylethylene carbonate, and4,5-difluoro-4,5-diallylethylene carbonate.

The fluorinated unsaturated cyclic carbonate may have any molecularweight that does not significantly impair the effects of the invention.The molecular weight is preferably 50 or higher and 500 or lower. Thefluorinated unsaturated cyclic carbonate having a molecular weightwithin this range can easily ensure the solubility of the fluorinatedunsaturated cyclic carbonate in the electrolyte solution and enableseasy achievement of the effects of the invention.

The fluorinated unsaturated cyclic carbonate may be produced by anymethod, and may be produced by any known method selected as appropriate.The molecular weight is more preferably 100 or higher and morepreferably 200 or lower.

The fluorinated unsaturated cyclic carbonates may be used alone or inany combination of two or more at any ratio.

The fluorinated unsaturated cyclic carbonate may be contained in anyamount that does not significantly impair the effects of the invention.

The amount of the fluorinated unsaturated cyclic carbonate is usuallypreferably 0.01% by mass or more, more preferably 0.1% by mass or more,still more preferably 0.5% by mass or more, while preferably 5% by massor less, more preferably 3% by mass or less, still more preferably 2% bymass or less, of 100% by mass of the electrolyte solution.

The fluorinated unsaturated cyclic carbonate in an amount within thisrange allows an electrochemical, device containing the electrolytesolution to exhibit an effect of sufficiently improving the cyclecharacteristics and can easily avoid a situation with reducedhigh-temperature storage characteristics, generation of a large amountof gas, and a reduced discharge capacity retention.

In order to effectively reduce burst or combustion of a battery in caseof overcharge, for example, of an electrochemical device containing theelectrolyte solution, the electrolyte solution of the invention maycontain an overcharge inhibitor.

Examples of the overcharge inhibitor include aromatic compounds such asbiphenyl, alkyl biphenyl, terphenyl, partially hydrogenated terphenyl,cyclohexyl benzene, t-butyl benzene, t-amyl benzene, diphenyl ether, anddibenzofuran; partially fluorinated aromatic compounds such as2-fluorobiphenyl, o-cyclohexyl fluorobenzene, and p-cyclohexylfluorobenzene; and fluoroanisole compounds such as 2,4-difluoroanisole,2,5-difluoroanisole, 2,6-difluoroanisole, and 3,5-difluoroanisole.

Preferred are aromatic compounds such as biphenyl, alkyl biphenyl,terphenyl, partially hydrogenated terphenyl, cyclohexyl benzene, t-butylbenzene, t-amyl benzene, diphenyl ether, and dibenzofuran.

These compounds may be used alone or in combination of two or more.

In order to achieve good balance between the overcharge inhibitingcharacteristics and the high-temperature storage characteristics with acombination use of two or more compounds, preferred is a combination ofcyclohexyl benzene and t-butyl benzene or t-amyl benzene, or acombination of at least one oxygen-free aromatic compound selected frombiphenyl, alkyl biphenyl, terphenyl, partially hydrogenated terphenyl,cyclohexyl benzene, t-butyl benzene, t-amyl benzene, and the like and atleast one oxygen-containing aromatic compound selected from diphenylether, dibenzofuran, and the like.

The electrolyte solution of the invention may further contain a knowndifferent aid.

Examples of the different, aid include carbonate compounds such aserythritan carbonate, spiro-bis-dimethylene carbonate, andmethoxyethyl-methyl carbonate; carboxylic anhydrides such as succinicanhydride, glutaric anhydride, maleic anhydride, citraconic anhydride,gintaconic anhydride, itaconic anhydride, diglycolic anhydride,cyclohexanedicarboxylic anhydride, cyclopentanetetracarboxylicdianhydride, and phenylsuccinic anhydride; spiro compounds such as2,4,8,10-tetraoxaspiro[5.5]undecane and3,9-divinyl-2,4,8,10-tetraoxaspiro[5.5]undecane; sulfur-containingcompounds such as ethylene sulfite, 1,3-propanesultone,1-fluoro-1,3-propanesultone, 2-fluoro-1,3-propanesultone,3-fluoro-1,3-propanesultone, 1-propane-1,3-sultone,1-fluoro-1-propene-1,3-sultone, 2-fluoro-1-propene-1,3-sultone,3-fluoro-1-propene-1,3-sultone, 1,4-butanesultone, 1-butene-1,4-sultone,3-butene-1,4-sultone, methyl fluorosulfonate, ethyl fluorosulfonate,methyl methanesulfonate, ethyl methanesulfonate, busulfan, sulfolene,diphenyl sulfone, N,N-dimethylmethanesulfonamide,N,N-diethylmethanesulfonamide, methyl vinyl sulfonate, ethyl vinylsulfonate, allyl vinyl sulfonate, propargyl vinyl sulfonate, methylallyl sulfonate, ethyl allyl sulfonate, allyl allyl sulfonate, propargylallyl sulfonate, and 1,2-bis(vinylsulfonyloxy)ethane;nitrogen-containing compounds such as 1-methyl-2-pyrrolidinone,1-methyl-2-piperidone, 3-methyl-2-oxazolidinone,1,3-dimethyl-2-imidazolidinone, and N-methylsuccinimide;phosphorus-containing compounds such as trimethyl phosphite, triethylphosphite, triphenyl phosphite, tximethyl phosphate, triethyl phosphate,triphenyl phosphate, dimethyl methyl phosphonate, diethyl ethylphosphonate, dimethyl vinyl phosphonate, diethyl vinyl phosphonate,ethyl diethyl phosphonoacetate, methyl dimethyl phosphinate, ethyldiethyl phosphinate, trimethylphosphine oxide, and triethylphosphxneoxide; hydrocarbon compounds such as heptane, octane, nonane, decane,and cycloheptane; and fluorine-containing aromatic compounds such asfluorobenzene, difluorobenzene, hexafluorobenzene, and benzotrifluoride.

These compounds may be used alone or in combination of two or more.

These aids can improve the capacity retention characteristics and thecycle characteristics after high-temperature storage.

The different aid may be used in any amount that does not significantlyimpair the effects of the invention.

The amount of the different aid is preferably 0.01% by mass or more and5% by mass or less of 100% by mass of the electrolyte solution.

The different aid in an amount within this range can easily sufficientlyexhibit the effects thereof and can easily avoid a situation withreduction in battery characteristics such as high-load dischargecharacteristics.

The amount of the different aid is more preferably 0.1% by mass or more,still more preferably 0.2% by mass or more, while more preferably 3% bymass or less, still more preferably 1% by mass or less.

The electrolyte solution of the invention may contain an acid anhydridehaving a succinic acid skeleton. Examples of the acid anhydride having asuccinic acid skeleton include succinic anhydride, maleic anhydride,citraconic acid, 2-methylsuccinic acid, 2,3-dimethylsuccinic acid,2-fluorosuccinic acid, and 2,3-difluorosuccinic acid. Preferred amongthese is succinic anhydride or maleic anhydride.

The acid anhydride having a succinic acid skeleton is preferably presentin an amount of 0.1 to 10% by mass, more preferably 0.5% by mass ormore, while more preferably 5% by mass or less, in the electrolytesolution.

The electrolyte solution of the invention may contain a cyclic sulfonicacid compound. Examples of the cyclic sulfonic acid compound include1,3-propanesultone, 1,4-butanesultone, 1-fluoro-1,3-propanesultone,2-fluoro-1,3-propanesultone, and 3-fluoro-1,3-propanesultone. In orderto improve the high-temperature characteristics, the electrolytesolution of the invention preferably contains 1,3-propanesultone and/or1,4-butanesultone.

The amount of the cyclic sulfonic acid compound is preferably 0.1 to 10%by mass, more preferably 0.5% by mass or more, while more preferably 5%by mass or less, in the electrolyte solution.

The electrolyte solution of the invention may further contain any of acyclic carboxylate, an ether compound, a nitrogen-containing compound, aboron-containing compound, an organosilicon-containing compound, afireproof agent (flame retardant), a surfactant, an additive forincreasing the permittivity, and an improver for cycle characteristicsand rate characteristics, to the extent that does not impair the effectsof the invention.

Examples of the cyclic carboxylate include those having a carbon numberof 3 to 12 in total in the structural formula. Specific examples thereofinclude gamma-butyrolactone, gamma-valerolactone, garama-caprolactone,and epsilon-caprolactone. In order to improve the characteristics of anelectrochemical device owing to improvement in the degree ofdissociation of lithium ions, particularly preferred isgamma-butyrolactone.

In general, the cyclic carboxylate is preferably present in an amount of0.1% by mass or more, more preferably 1% by mass or more, of 100% bymass of the solvent. The cyclic carboxylate in an amount within thisrange can easily improve the electric conductivity of the electrolytesolution, improving the large-current discharge characteristics of anelectrochemical device. The amount of the cyclic carboxylate is alsopreferably 10% by mass or less, more preferably 5% by mass or less. Suchan upper limit may allow the electrolyte solution to have a viscositywithin an appropriate range, may make it possible to avoid a reductionin the electric conductivity, may reduce an increase in the resistanceof the negative electrode, and may allow the electrochemical device tohave large-current discharge characteristics within a favorable range.

The cyclic carboxylate to be suitably used may also be a fluorinatedcyclic carboxylate (fluorine-containing lactone). Examples of thefluorine-containing lactone include fluorine-containing lactonesrepresented by the following formula (E):

wherein X¹⁵ to X²⁰ are the same as or different from each other, and areeach —B, —F, —C1, —CH₃, or a fluorinated alkyl group; and at least oneselected from X¹⁵ to X²⁰ is a fluorinated alkyl group.

Examples of the fluorinated alkyl group for X¹⁵ to X²⁰ include —CFH₂,—CF₂H, —CF₃, —CH₂CF₃, —CF₂CF₃, —CH₂CF₂CF₃, and —CF(CF₃)₂. In order toachieve high oxidation resistance and an effect of improving the safety,—CH₂CF₃ and —CH₂CF₂CF₃ are preferred.

One of X¹⁵ to X²⁰ or a plurality thereof may be replaced by —H, —F, —Cl,—CH₃, or a fluorinated alkyl group only when at least one selected fromX¹⁵ to X²⁰ is a fluorinated alkyl group. In order to give goodsolubility of an electrolyte salt, the number of substituents ispreferably 1 to 3, more preferably 1 or 2.

The substitution of the fluorinated alkyl group may be at any of theabove sites. In order to give a good synthesizing yield, thesubstitution site is preferably X¹⁷ and/or X¹⁸. In particular, X¹⁷ orX¹⁸ is preferably a fluorinated alkyl group, especially —CH₂CF₃ or—CH₂CF₂CF₃. The substituent for X¹⁵ to X²⁰ other than the fluorinatedalkyl group is —H, —F, —Cl, or CH₃. In order to give good solubility ofan electrolyte salt, —H is preferred.

In addition to those represented by the above; formula, thefluorine-containing lactone may also be a fluorine-containing lactonerepresented by the following formula (F):

wherein one of A or B is CX²⁶X²⁷ (where X²⁶ and X²⁷ are the same as ordifferent from each other, and are each —H, —F, —Cl, —CF₃, —CH₃, or analkylene group in which a hydrogen atom is optionally replaced by ahalogen atom and which optionally contains a hetero atom in the chain)and the other is an oxygen atom; Rf¹² is a fluorinated alkyl group orfluorinated alkoxy group optionally containing an ether bond; X²¹ andX²² are the same as or different from each other, and are each —H, —F,—Cl, —CF₃, or CH₃; X²³ to X²³ are the same as or different from eachother, and are each —H, —F, —Cl, or an alkyl group in which a hydrogenatom is optionally replaced, by a halogen atom and which optionallycontains a hetero atom in the chain; and n=0 or 1.

A preferred example of the fluorine-containing lactone represented bythe formula (F) is a 5-membered ring structure represented by thefollowing formula (G):

(wherein A, B, Rf¹², X²¹, X²², and X²³ are defined as in the formula(F)) because it can be easily synthesized and can have good chemicalstability. Further, in relation to the combination of A and B,fluorine-containing lactones represented by the following formula (H);

(wherein Rf¹², X²¹, X²², X²³, X²⁶, and X²⁷ are defined as in the formula(F)) and fluorine-containing lactones represented by the followingformula (I):

(wherein Rf¹², X²¹, X²², X²³, X²⁶, and X²⁷ are defined as in the formula(F)) may be mentioned.

In order to particularly give excellent characteristics such as highpermittivity and high withstand voltage, and to improve thecharacteristics of the electrolyte solution in the invention, forexample, to give good solubility of an electrolyte salt and to reducethe internal resistance well, those represented by the followingformulae:

may be mentioned.

The presence of a fluorinated cyclic carboxylate can lead to, forexample, effects of improving the ion conductivity, improving thesafety, and improving the stability at high temperature.

The ether compound is preferably a C3-C10 acyclic ether or a C3-C6cyclic ether.

Examples of the C3-C10 acyclic ether include diethyl ether, di-n-butylether, dimethoxymethane, methoxyethoxymethane, diethoxymethane,dimethoxyethane, methoxyethoxyethane, diethoxyethane, ethylene glycoldi-n-propyl ether, ethylene glycol di-n-butyl ether, and diethyleneglycol dimethyl ether.

Further, the ether compound may also suitably be a fluorinated ether.

An example of the fluorinated ether is a fluorinated ether (K)represented by the following formula (K):

Rf¹—O—Rf²   (K)

(wherein Rf¹ and Rf² are the same as or different from each other, andare each a C1-C10 alkyl group or a C1-C10 fluorinated alkyl group; andat least one selected from. Rf¹ and Rf² is a fluorinated alkyl group).The presence of the fluorinated ether (K) allows the electrolytesolution to have improved Incombustibility as well as improved stabilityand safety at high temperature under high voltage.

In the formula (K), at least one selected from Rf¹ and Rf² is a C1-C10fluorinated alkyl group. In order to allow the electrolyte solution tohave further improved incombustibility and further improved stabilityand safety at high temperature under high voltage, both Rf¹ and Rf² arepreferably C1-C10 fluorinated alkyl groups. In this case, Rf¹ and Rf²may be the same as or different from each other.

Particularly preferably, Rf¹ and Rf² are the same as or different fromeach other, and Rf¹ is a C3-C6 fluorinated alkyl group and Rf² is aC2-C6 fluorinated alkyl group.

If the sum of the carbon numbers of Rf¹ and Rf² is too small, thefluorinated ether may have too low a boiling point. Too large a carbonnumber of Rf¹ or Rf² may cause low solubility of an electrolyte salt,may start to adversely affect the miscibility with other solvents, andmay cause high viscosity, resulting in poor rate characteristics. Inorder to achieve an excellent boiling point and rate characteristics,advantageously, the carbon number of Rf¹ is 3 or 4 and the carbon numberof Rf² is 2 or 3.

The fluorinated ether (K) preferably has a fluorine content of 40 to 75%by mass. The fluorinated ether (K) having a fluorine content within thisrange may lead to particularly excellent balance between thenon-flammability and the miscibility. The above range is also preferredfor good oxidation resistance and safety.

The lower limit of the fluorine content is more preferably 45% by mass,still more preferably 50% by mass, particularly preferably 55% by mass.The upper limit thereof is more preferably 70% by mass, still morepreferably 66% by mass.

The fluorine content of the fluorinated ether (K) is a value calculatedbased on the structural formula of the fluorinated ether (K) by thefollowing formula:

{(Number of fluorine atoms×19)/(Molecular weight of fluorinated ether(K))}×100(%).

Examples of Rf¹ include CF₃CF₂CH₂—, CF₃CFHCF₂—, HCF₂CF₂CF₂—,HCF₂CF₂CH₂—, CF₃CF₂CH₂CH₂—, CF₃CFHCF₂CH₂—, HCF₂CF₂CF₂CF₂—,HCF₂CF₂CF₂CH₂—, HCF₂CF*CH₂CH₂—, and HCF₂CF(CF₃)CH₂—.

Examples of Rf² include CF₃CF₂CH₂—, CF₃CFHCF₂—, CF₂HCF₂CF₂—,CF₂HCF₂CH₂—, CF₃CF₂CH₂CH₂—, CF₃CFHCF₂CH₂—, CF₂HCF₂CF₂CF₂—,CF₂HCF₂CF₂CH₂—, CF₂HCF₂CH₂CH₂—, CF₂HCF(CF₃)CH₂—, CF₂HCF₂—, CF₂HCH₂—, andCH₃CF₂—.

Specific examples of the fluorinated ether (K) includeHCF₂CF₂CH₂OCF₂CF₂H, CF₃CF₂CH₂OCF₂CF₂H, HCF₂CF₂CH₂OCF₂CFHCF₃,CF₃CF₂CH₂OCF₂CFHCF₃, C₆F₁₃OCH₃, C₆F₁₃OC₂H₅, C₈F₁₇OCH₃, C₈F₁₇OC₂H₅,CF₃CFHCF₂CH(CH₃)OCF₂CFHCF₃, HCF₂CF₂OCH(C₂H₅)₂, HCF₂CF₂OC₄H₉,HCF₂CF₂OCH₂CH(C₂H₅)₂, and HCF₂CF_(2d)CCH₂CH(CH₃)₂.

In particular, those having HCF₂— or CF₃CFH— at one or each end canprovide a fluorinated ether (K) having excellent polarizability and ahigh boiling point. The boiling point of the fluorinated ether (K) ispreferably 67° C. to 120° C., more preferably 80° C or higher, stillmore preferably 90° C. or higher.

Such a fluorinated ether (K) may include one or two or more ofCF₃CH₂OCF₂CFHCF₃, CF₃CF₂CH₂OCF₂CFHCF₃, HCF₂CF₂CH₂OCF₂CFHCF₃,HCF₂CF₂CH₂OCH₂CF₂CF₂H, CF₃CFHCF₂CH₂OCF₂CFHCF₃, HCF₂CF₂CH₂OCF₂CF₂H,CF₃CF₂CH₂OCF₂CF₂H, and the like.

Advantageously, in order to achieve a high boiling-point and goodmiscibility with other solvents and to give good solubility of anelectrolyte salt, the fluorinated ether (K) is preferably at least oneselected from the group consisting of HCF₂CF₂CH₂OCF₂CFHCF₃ (boilingpoint: 106° C.), CF₃CF₂CH₂OCP₂CFHCF₃ (boiling point: 82° C.),HCF₂CF₂CH₂OCF₂CF₂H (boiling point: 92° C.), and CF₃CF₂CH₂OCF₂CF₂H(boiling point: 68° C.), more preferably at least one selected from thegroup consisting of HCF₂CF₂CH₂OCF₂CFHCF₃ (boiling point: 106° C.), andHCF₂CF₂CH₂OCF₂CF₂H (boiling point; 92° C).

Examples of the C3-C6 cyclic ether include 1,3-dioxane,2-methyl-1,3-dioxane, 4-methyl-1,3-dioxane, 1,4-dioxane, and fluorinatedcompounds thereof. In order to achieve a high ability to solvate withlithium ions and improve the degree of ion dissociation, preferred aredimethoxymethane, diethoxymethane, ethoxymethoxymethane, ethylene glycoln-propyl ether, ethylene glycol di-n-butyl ether, and diethylene glycoldimethyl ether. In order to achieve low viscosity and to give a high ionconductivity, particularly preferred are dimethoxymethane,diethoxymethane, and ethoxymethoxymethane.

Examples of the nitrogen-containing compound include nitrile,fluorine-containing nitrile, carboxylic acid amide, fluorine-containingcarboxylic acid amide, sulfonic acid amide, and fluorine-containingsulfonic acid amide. Also, 1-methyl-2-pyrrolidinone,1-methyl-2-piperidone, 3-methyl-2-oxazilidinone,1,3-dimethyl-2-imidazoiidinone, and N-methylsuccinimide may be used.

Examples of the boron-containing compound include borates such astrimethyl borate and triethyl borate, boric acid ethers, and alkylborates.

Examples of the organosilicon-containing compound include (CH₃)₄—Si and(CH₃)₃—Si—Si(CH₃)₃.

Examples of the fireproof agent (flame retardant) includeorganophosphates and phosphazene-based compounds. Examples of theorganophosphates include fluorine-containing alkyl phosphates,non-fluorine-containing alkyl phosphates, and aryl phosphates. In orderto achieve a flame retardant effect even in a small amount,fluorine-containing alkyl phosphates are particularly preferred.

Specific examples of the fluorine-containing alkyl phosphates includefluorine-containing dialkyl phosphates disclosed in JP H11-233141 A,cyclic alkyl phosphates i disclosed in JP H11-283669 A, andfluorine-containing trialkyl phosphates.

Preferred examples of the fireproof agent (flame retardant) include(CH₃O)₃P═O, (CF₃CH₂O)₃P═O, (HCF₂CH₂O)₃P═O, (CF₃CF₂CH₂)₃P═O, and(HCF₂CF₂CH₂)₃P═O.

The surfactant may be any of cationic surfactants, anionic surfactants,nonionic surfactants, and amphoteric surfactants. In order to give goodcycle characteristics and rate characteristics, the surfactant ispreferably one containing a fluorine atom.

Preferred examples of such a surfactant containing a fluorine atominclude fluorine-containing carboxylic acid s salts represented by thefollowing formula:

Rf¹COO⁻M⁺

(wherein Rf¹ is a C3-C10 fluorinated alkyl group optionally containingan ether bond; M⁺ is Li⁺, Na⁺, K⁺, or NHR′₃ ⁺ , wherein R's are the sameas or different from each other, and are each H or a C1-C3 alkyl group),and fluorine-containing sulfonic acid salts represented by the followingformula:

Rf²SO₃ ⁻M⁺

(wherein Rf² is a C3-C10 fluorinated alkyl group optionally containingan ether bond; M⁺ is Li⁺, Na⁺, K⁺, or NMR′₃ ⁺, wherein R's are the sameas or different from each other, and are each H or a C1-C3 alkyl group).

In order to reduce the surface tension of the electrolyte solutionwithout impairing the charge and discharge cycle characteristics, thesurfactant is preferably present in an amount of 0.01 to 2% by mass inthe electrolyte solution.

Examples of the additive for increasing the permittivity includesulfolane, methyl sulfolane, γ-butyrolactone, γ-valerolactone,acetonitrile, and propionitrile.

Examples of the improver for cycle characteristics and ratecharacteristics include methyl acetate, ethyl acetate, tetrahydrofuran,and 1,4-dioxane.

The electrolyte solution of the invention may be combined with a polymermaterial and thereby formed into a gel-like (plasticized), gelelectrolyte solution.

Examples of such a polymer material include conventionally knownpolyethylene oxide and polypropylene oxide, and modified productsthereof (see JP H08-222270 A, JP 2002-100405 A); polyacrylate-basedpolymers, polyacrylonitrile, and fluororesins such as polyvinylidenefluoride and vinylidene fluoride-hexafluoropropylene copolymers (see JPH04-506726 T, JP H08-507407 T, JP H10-294131 A) ; and composites of anyof these fluororesins and any hydrocarbon resin (see JP H11-35765 A, JPH11-86630 A). In particular, polyvinylidene fluoride or a vinylidenefluoride-hexafluoropropylene copolymer is preferably used as a polymermaterial for a gel electrolyte.

The electrolyte solution of the invention may also contain an ionconductive compound disclosed in Japanese Patent Application No.2004-301934.

This ion conductive compound is an amorphous fluorine-containingpolyether compound having a fluorine-containing group at a side chainand is represented by the following formula (1-1):

A-(D)-B   (1-1)

wherein D is represented by the following formula (2-1):

-(D1)_(n)-(FAE)_(n)-(AE)_(p)-(Y)_(q)-   (2-1)

[wherein D1 is an ether unit containing a fluorine-containing ethergroup at a side chain and is represented by the following formula (2a):

(wherein Rf is a fluorine-containing ether group optionally containing acrosslinkable functional group; and R¹⁰ is a group or a bond that linksRf and the main chain);

FAE is an ether unit containing a fluorinated alkyl group at a sidechain and is represented by the following formula (2b):

(wherein Rfa is a hydrogen atom or a fluorinated alkyl group optionallycontaining a crosslinkable functional group; and R¹¹ is a group or abond that links Rfa and the main chain);

AE is an ether unit represented by the following formula (2c):

(wherein R¹³ is a hydrogen atom, an alkyl group optionally containing acrosslinkable functional group, an aliphatic cyclic hydrocarbon groupoptionally containing a crosslinkable functional group, or an aromatichydrocarbon group optionally containing a crosslinkable functionalgroup; and R¹² is a group or a bond that links R¹³ and the main chain);

Y is a unit containing at least one selected from the following formulae(2d-1) to (2d-3):

n is an integer of 0 to 200;

m is an integer of 0 to 200;

p is an integer of 0 to 10000;

q is an integer of 1 to 100;

n+m is not 0; and

the bonding order of D1, FAS, AE, and Y is not specified]; and

A and B are the same as or different from each other, and are each ahydrogen atom, an alkyl group optionally containing a fluorine atomand/or a crosslinkable functional group, a phenyl group optionallycontaining a fluorine atom and/or a crosslinkable functional group, a—COOH group, —OR (where R is a hydrogen atom or an alkyl groupoptionally containing a fluorine atom and/or a crosslinkable functionalgroup), an ester group, or a carbonate group, and when an end of D is anoxygen atom, A and B are each none of a —COOH group, —OR, an estergroup, and a carbonate group.

The electrolyte solution of the invention may further contain adifferent additive, if necessary. Examples of the different additiveinclude metal oxides and glass.

The electrolyte solution of the invention preferably contains 0.5 to 70ppm of HF. The presence of HF can promote formation of a film of theadditive. Too small an amount of HF tends to impair the ability of theadditive to form a film on the negative electrode, impairing thecharacteristics of an electrochemical device. Too large an amount of HFtends to impair the oxidation resistance of the electrolyte solution dueto the influence by HF. The electrolyte solution of the invention, evenwhen containing HF in an amount within the above range, causes noreduction in capacity recovery in high-temperature storage of anelectrochemical device.

The amount of HF is more preferably 1 ppm or more, still more preferably2.5 ppm or more. The amount of HF is also more preferably 60 ppm orless, still more preferably 50 ppm or less, particularly preferably 30ppm or less.

The amount of HF can be determined by neutralization titration.

The electrolyte solution of the invention is preferably prepared by anymethod using the aforementioned components.

The electrolyte solution of the invention can be suitably applied toelectrochemical devices such as secondary batteries, lithium-ionsecondary batteries, and electric double layer capacitors. Such anelectrochemical device including the electrolyte solution ox theinvention is also one aspect of the invention.

Examples of the electrochemical devices include lithium-ion secondarybatteries, capacitors (electric double-layer capacitors), radicalbatteries, solar cells (in particular, dye-sensitized solar cells), fuelcells, various electrochemical sensors, electrochromic elements,electrochemical switching elements, aluminum electrolytic capacitors,and tantalum electrolytic capacitors. Preferred are lithium-ionsecondary batteries and electric double-layer capacitors.

A module including the electrochemical device is also one aspect of theinvention.

The invention also relates to a secondary battery including theelectrolyte solution of the invention. The secondary battery may be alithium ion secondary battery. The following describes an exemplary casein which the electrochemical device or the secondary battery of theinvention is a lithium ion secondary battery or an electric double-layercapacitor.

The lithium-ion secondary battery may include a positive electrode, anegative electrode, and the above electrolyte solution.

<Positive Electrode>

The positive electrode includes a positive electrode active materiallayer containing a positive electrode active material and a currentcollector.

The positive electrode active material may be any material that canelectrochemically occlude and release lithium ions. For example, asubstance containing lithium and at least one transition metal ispreferred. Specific examples thereof include lithium-containingtransition i metal complex oxides and lithium-containing transitionmetal phosphoric acid compounds. In particular, the positive electrodeactive material is preferably a lithium-containing transition metalcomplex oxide that generates high voltage.

The transition metal of the lithium-containing transition metal complexoxide is preferably V, Ti, Cr, Mn, Fe, Co, Ni, Cu, or the like. Specificexamples thereof include lithium-cobalt complex oxides such as LiCoO₂,lithium-nickel complex oxides such as LiNiO₂, lithium-manganese complexoxides such as LiMnO₂, LiMn₂O₄, and Li₂MnO₄, and those obtained bysubstituting some of transition metal atoms as main components of theselithium transition metal complex oxides with another element such as Na,K, B, F, Al, Ti, V, Cr, Mn, Fe, Co, Li, Ni, Cu, Zn, Mg, Ga, Zr, Si, Kb,Mo, Sn, or W. Specific examples of those obtained by substitutioninclude LiNi_(0.5)Mn_(0.5)O₂, LiNi_(0.85)Co_(0.10)Al_(0.05)O₂,LiNi_(0.5)Co_(0.2)Mn_(0.3)O₂m LiNi_(0.6)Co_(0.2)Mn_(0.2)O₂,LiNi_(0.33)Co_(0.33)Mn_(0.33)O₂, LiNi_(0.45)Co_(0.10)Al_(0.45)O₂,LiMn_(1.8)Al_(0.2)O₄, and LiMn_(1.5)Ni_(0.5)O₄.

The lithium-containing transition metal complex oxide is preferably anyof LiMn_(1.5)Ni_(0.5)O₄, LiNi_(0.5)Co_(0.2)Mn_(0.3)O₂, andLiNi_(0.6)Co_(0.2)Mn_(0.2)O₂ each of which has a high energy densityeven, at high voltage.

The transition metal of the lithium-containing transition metalphosphoric acid compound is preferably V, Ti, Cr, Mn, Fe, Co, Ni, Cu, orthe like. Specific examples thereof include iron phosphates such asLiFePO₄, Li₃Fe₂(PO₄)₃, and LiFeP₂O₇, cobalt phosphates such as LiCoPO₄,and those obtained by substituting some of transition metal atoms asmain components of these lithium transition metal phosphoric acidcompounds with another element such as Al, Ti, V, Cr, Mn, Fe, Co, Li,Ni, Cu, Zn, Mg, Ga, Zr, Nb, or Si.

Examples of the lithium-containing transition metal complex oxideinclude

lithium-manganese spinel complex oxides represented by the formula:Li_(a)Mn_(2-b)M¹ _(b)O₄ (wherein 0.9≤a; 0≤b≤1.5; and M¹ is at least onemetal selected from the group consisting of Fe, Co, Ni, Cu, Zn, Al, Sn,Cr, V, Ti, Mg, Ca, Sr, B, Ga, In, Si, and Ge),

lithium-nickel complex oxides represented by the formula: LiNi_(1-o)M²_(c)O₂ (wherein 0≤c≤0.5; and M² is at least one metal selected from thegroup consisting of Fe, Co, Mn, Cu, Zn, Al, Sn, Cr, V, Ti, Mg, Ca, Sr,B, Ga, In, Si, and Ge), and

lithium-cobalt complex oxides represented by the formula: LiCo_(1-d)M³_(d)O₂ (wherein 0≤d≤0.5; and M³ is at least one metal selected from thegroup consisting of Fe, Ni, Mn, Cu, Zn, Al, Sn, Cr, V, Ti, Mg, Ca, Sr,B, Ga, In, Si, and Ge).

In order to provide a high-power lithium-ion secondary battery having ahigh energy density, preferred is LiCoO₂, LiMnO₂, LiNiO₂, LiMn₂O₄,LiNi_(0.8)Co_(0.15)Al_(0.05)O₂, or LiMi_(1/3)CO_(1/3)Mn_(1/3)O₂.

Other examples of the positive electrode active material includeLiFePO₄, LiNi_(0.8)Co_(0.2)O₂, Li_(1.2)Fe_(0.4)Mn_(0.4)O₂,LiNi_(0.5)Mn_(0.5)O₂, and LiV₃O₆.

In order to improve the continuous charge characteristics, the positiveelectrode active material preferably contains lithium phosphate. Lithiumphosphate may be used in any manner, and is preferably used in admixturewith the positive electrode active material. The lower limit of theamount of lithium phosphate used is preferably 0.1% by mass or more,more preferably 0.3% by mass or more, still more preferably 0.5% by massor more, relative to the sum of the amounts of the positive electrodeactive material and lithium phosphate. The upper limit thereof ispreferably 10% by mass or less, more preferably 8% by mass or less,still more preferably 5% by mass or less.

To a surface of the positive electrode active material may be attached asubstance having a composition different from the positive electrodeactive material. Examples of the substance attached to the surfaceinclude oxides such as aluminum oxide, silicon oxide, titanium oxide,zirconium oxide, magnesium oxide, calcium oxide, boron oxide, antimonyoxide, and bismuth oxide; sulfates such as lithium sulfate, sodiumsulfate, potassium sulfate, magnesium sulfate, calcium sulfate, andaluminum sulfate; carbonates such as lithium carbonate, calciumcarbonate, and magnesium carbonate; and carbon.

Such a substance may be attached to a surface of the positive electrodeactive material by, for example, a method of dissolving or suspendingthe substance in a solvent, impregnating the solution or suspension intothe positive electrode active material, and drying the impregnatedmaterial; a method of dissolving or suspending a precursor of thesubstance in a solvent, impregnating the solution or suspension into thepositive electrode active material, and heating the material and theprecursor to cause a reaction therebetween; or a method of adding thesubstance to a precursor of the positive electrode active material andsimultaneously sintering the materials. In the case of attaching carbon,for example, a carbonaceous material in the form of activated carbon maybe mechanically attached to the surface afterward.

For the amount of the substance attached to the surface in terms of themass relative to the amount of the positive electrode active material,the lower limit thereof is preferably 0.1 ppm or more, more preferably 1ppm or more, still more preferably 10 ppm or more, while the upper limitthereof is preferably 20% or less, more preferably 10% or less, stillmore preferably 5% or less. The substance attached to the surface canreduce oxidation of the electrolyte solution on the surface of thepositive electrode active material, improving the battery life. Toosmall an amount of the substance may fail to sufficiently provide thiseffect. Too large an amount thereof may hinder the entrance and exit oflithium ions, increasing the resistance.

Particles of the positive electrode active material may have any shapeconventionally used, such as a bulky shape, a polyhedral shape, aspherical shape, an ellipsoidal shape, a plate shape, a needle shape, ora pillar shape. The primary particles may agglomerate to form secondaryparticles.

The positive electrode active material has a tap density of preferably0.5 a/cm³ or higher, more preferably 0.8 g/cm³ or higher, still morepreferably 1.0 g/cm³ or higher. The positive electrode active materialhaving a tap density below the lower limit may cause an increased amountof a dispersion medium required and increased amounts of a conductivematerial and a binder required in formation of the positive electrodeactive material layer, as well as limitation on the packing fraction ofthe positive electrode active material in the positive electrode activematerial layer, resulting in limitation on the battery capacity. Acomplex oxide powder having a high tap density enables formation of apositive electrode active material layer with a high density. The tapdensity is preferably as high as possible and has no upper limit, ingeneral. Still, too high a tap density may cause diffusion of lithiumions in the positive electrode active material layer with theelectrolyte solution serving as a diffusion medium to function as arate-determining step, easily impairing the load characteristics. Thus,the upper limit of the tap density is preferably 4.0 g/cm³ or lower,more preferably 3.7 g/cm³ or lower, still more preferably 3.5 g/cm³ orlower.

In the invention, the tap density is determined as a powder packingdensity (tap density) g/cm³ when 5 to 10 g of the positive electrodeactive material powder is packed into a 10-ml glass graduated cylinderand the cylinder is tapped 200 times with a stroke of about 20 mm.

The particles of the positive electrode active material, have a mediansize d50 (or a secondary particle size when the primary particlesagglomerate to form secondary particles) of preferably 0.3 μm orgreater, more preferably 0.5 μm or greater, still more preferably 0.8 μmor greater, most preferably 1.0 μm or greater, while preferably 30 μm orsmaller, more preferably 27 μm or smaller, still more preferably 25 μmor smaller, most preferably 22 μm or smaller. The particles having amedian size below the lower limit may fail to provide a product, with ahigh tap density. The particles having a median size greater than theupper limit may cause prolonged diffusion of lithium in the particles,impairing the battery performance and generating streaks in formation ofthe positive electrode for a battery, i.e., when the active material andcomponents such as a conductive material and a binder are formed intoslurry by adding a solvent and the slurry is applied in the form of afilm, for example. Mixing two or more positive electrode activematerials having different median sizes d50 can further improve theeasiness of packing in formation of the positive electrode.

In the invention, the median size d50 is determined using a known laserdiffraction/scattering particle size distribution analyzer. In the caseof using LA-920 (Horiba, Ltd.) as the particle size distributionanalyzer, the dispersion medium used in the measurement is a 0.1% bymass sodium hexametaphosphate aqueous solution and the measurementrefractive index is set to 1.24 after 5-minute ultrasonic dispersion.

When the primary particles agglomerate to form secondary particles, theaverage primary particle size of the positive electrode active materialis preferably 0.05 μm or greater, more preferably 0.1 μm or greater,still more preferably 0.2 μm or greater. The upper limit thereof ispreferably 5 μm or smaller, more preferably 4 μm or smaller, still morepreferably 3 μm or smaller, most preferably 2 μm or smaller. The primaryparticles having an average primary particle size greater than the upperlimit may have difficulty in forming spherical secondary particles,adversely affecting the powder packing. Further, such primary particlesmay have a greatly reduced specific surface area, highly possiblyimpairing the battery performance such as output characteristics. Incontrast, the primary particles having an average primary particle sizebelow the lower limit may usually be insufficiently grown crystals,causing poor charge and discharge reversibility, for example.

In the invention, the primary particle size is measured by scanningelectron microscopic (SEM) observation, Specifically, the primaryparticle size is determined as follows. A photograph at a magnificationof 1000× is first taken. Any 50 primary particles are selected and themaximum length between the left and right boundary lines of each primaryparticle is measured along the horizontal line. Then, the average valueof the maximum lengths is calculated, which is defined as the primaryparticle size.

The positive electrode active material has a BET specific surface areaof preferably 0.1 m²/g or larger, more preferably 0.2 m²/g or larger,still more preferably 0.3 m²/g or larger, while preferably 50 m²/g orsmaller, more preferably 40 m²/g or smaller, still more preferably 30m²/g or smaller. The positive electrode active material having a BETspecific surface area smaller than the above range may easily impair thebattery performance. The positive electrode active material having a BETspecific surface area larger than the above range may less easily havean increased tap density, easily causing a difficulty in applying thematerial in formation of the positive electrode active material layer.

In the invention, the BET specific surface area is defined by a valuedetermined by single point BET nitrogen adsorption utilizing a gas flowmethod using a surface area analyzer (e.g., fully automatic surface areameasurement device, Oiikura Riken Co., Ltd.), a sample pre-dried innitrogen stream at 150° C. for 30 minutes, and a nitrogen-helium gasmixture with the nitrogen pressure relative to the atmospheric pressurebeing accurately adjusted to 0.3.

When the lithium-ion secondary battery is used as a large-sizelithium-ion secondary battery for hybrid vehicles or distributedgeneration, it needs to achieve high output. Thus, the particles of thepositive electrode active material preferably mainly composed ofsecondary particles.

The particles of the positive electrode active material preferablyinclude 0.5 to 7.0% by volume of fine particles having an averagesecondary particle size of 40 μm or smaller and having an averageprimary particle size of 1 μm or smaller. The presence of fine particleshaving an average primary particle size of 1 μm or smaller enlarges thecontact area with the electrolyte solution and enables more rapiddiffusion of lithium ions between the electrode and the electrolytesolution, improving the output performance of the battery.

The positive electrode active material may be produced by any usualmethod of producing an inorganic compound. In particular, a spherical,or ellipsoidal active material can be produced by various methods. Forexample, a material substance of transition metal is dissolved orcrushed and dispersed in a solvent such as water, and the pH of thesolution or dispersion is adjusted under stirring to form a sphericalprecursor. The precursor is recovered and, if necessary, dried. Then, aLi source such as LiOH, Li₂CO₃, or LiNO₃ is added thereto and themixture is sintered at high temperature, thereby providing an activematerial.

In production of the positive electrode, the aforementioned positiveelectrode active materials may be used alone or in any combination oftwo or more thereof having different compositions at any ratio.Preferred examples of the combination in this case include a combinationof LiCoO₂ and LiMn₂O₄ in which part of Mn may optionally be replaced bya different transition metal (e.g., LiNi_(0.33)Co_(0.33)Mn_(0.33)O₂),and a combination with LiCoO₂ in which part of Co may optionally bereplaced by a different transition metal.

In order to achieve a high battery capacity, the amount of the positiveelectrode active material is preferably 50 to 99% by mass, morepreferably 80 to 99% by s mass, of the positive electrode mixture. Theamount of the positive electrode active material in the positiveelectrode active material layer is preferably 80% by mass or more, morepreferably 82% by mass or more, particularly preferably 84% by mass ormore, while preferably 99% by mass or less, more preferably 98% by massor less. Too small an amount of the positive electrode active materialin the positive electrode active material layer may cause aninsufficient electric capacity. In contrast, too large an amount thereofmay cause insufficient strength of the positive electrode.

The positive electrode mixture preferably further contains a binder, athickening agent, and a conductive material.

The binder may be any material that is safe against a solvent to be usedin production of the electrode and the electrolyte solution. Examplesthereof include polyvinylidene fluoride, polytetrafluoroethylene,polyethylene, polypropylene, SBR (styrene-butadiene rubber), isoprenerubber, butadiene rubber, ethylene-acrylic acid copolymers,ethylene-methacrylic acid copolymers, polyethylene terephthalate,polymethyl methacrylate, polylmide, aromatic polyamide, cellulose, nitrocellulose, NBR (acrylonitrile-butadiene rubber), fluoroelastomers,ethylene-propylene rubber, styrene-butadiene-styrene block copolymersand hydrogenated products thereof, EPDM (ethylene-propylene-dieneterpolymers), styrene-ethylene-butadiene-ethylene copolymers,styrene-isoprene-styrene block copolymers and hydrogenated productsthereof, syndiotactic-1,2-polybutadiene, polyvinyl acetate,ethylene-vinyl acetate copolymers, propylene-α-olefin copolymers,fluorinated polyvinylidene fluoride, tetrafluoroethylene-ethylenecopolymers, and polymer compositions having ion conductivity of alkalimetal ions (especially, lithium ions). These substances may be usedalone or in any combination of two or more at any ratio.

The amount of the binder, which is expressed as the proportion, of thebinder in the positive electrode active material layer, is usually 0.1%by mass or more, preferably 1% by mass or more, more preferably 1.5% bymass or more. The proportion is also usually 80% by mass or less,preferably 60% by mass or less, still more preferably 40% by mass orless, most preferably 10% by mass cr less. Too low a proportion of thebinder may fail to sufficiently hold the positive electrode activematerial and cause insufficient mechanical strength of the positiveelectrode, impairing the battery performance such as cyclecharacteristics. In contrast, too high a proportion thereof may causereduction in battery capacity and conductivity.

Examples of the thickening agent include carboxymethyl cellulose, methylcellulose, hydroxymethyl cellulose, ethyl cellulose, polyvinyl alcohol,oxidized starch, monostarch phosphate, casein, and salts thereof. Theseagents may be used alone or in any combination of two or more at anyratio.

The proportion of the thickening agent relative to the active materialis usually 0.1% by mass or higher, preferably 0.2% by mass or higher,more preferably 0.3% by mass or higher, while usually 5% by mass orlower, preferably 3% by mass or lower, more preferably 2% by mass orlower. The thickening agent at a proportion lower than the above rangemay cause significantly poor easiness of application. The thickeningagent at a proportion higher than the above range may cause a lowproportion of the active material In the positive electrode activematerial layer, resulting in a low capacity of the battery and highresistance between the positive electrode active materials.

The conductive material may be any known conductive material. Specificexamples thereof include metal materials such as copper and nickel, andcarbon materials such as graphite, including natural graphite andartificial graphite, carbon black, including acetylene black, andamorphous carbon, including needle coke. These materials may be usedalone or in any combination of two or more at any ratio. The conductivematerial is used in an amount of usually 0.01% by mass or more,preferably 0.1% by mass or more, more preferably 1% by mass or more,while usually 50% by mass or less, preferably 30% by mass or less, morepreferably 15% by mass or less, in the positive electrode activematerial layer. The conductive material in an amount less than the aboverange may cause insufficient conductivity. In contrast, the conductivematerial in an amount more than the above range may cause a low batterycapacity.

The solvent for forming slurry may be any solvent that can dissolve ordisperse therein the positive electrode active material, the conductivematerial, and the binder, as well as a thickening agent used asappropriate. The solvent may be either an aqueous solvent or an organicsolvent. Examples of the aqueous medium include water and solventmixtures of an alcohol and water. Examples of the organic medium Includealiphatic hydrocarbons such as hexane; aromatic hydrocarbons such asbenzene, toluene, xylene, and methyl naphthalene; heterocyclic compoundssuch as quinoline and pyridine; ketones such as acetone, methyl ethylketone, and cyclohexanone; esters such as methyl acetate and methylacrylate; amines such as diethylene triamine and N,N-dimethylaminopropylamine; ethers such as diethyl ether, propylene oxide, andtetrahydrofuran (THF); amides such as N-methylpyrrolidone (NMP),dimethyl formamide, and dimethyl acetamide; and aprotic polar solventssuch as hexamethyl phospharamide and dimethyl sulfoxide.

Examples of the material of the current collector for a positiveelectrode include metal materials such as aluminum, titanium, tantalum,stainless steel, and nickel, and alloys thereof; and carbon materialssuch as carbon cloth and carbon paper. Preferred is any metal material,especially aluminum or am alloy thereof.

In the case of a metal material, the current collector may be in theform of metal foil, metal cylinder, metal, coil, metal plate, metalfilm, expanded metal, punched metal, metal, foam, or the like. In thecase of a carbon material, it may be in the form of carbon plate, carbonfilm, carbon cylinder, or the like. Preferred among these is a metalfilm. The film may be in the form of mesh, as appropriate. The film mayhave any thickness, and the thickness is usually 1 μm or greater,preferably 3 μm or greater, more preferably 5 μm or greater, whileusually 1 μ, or smaller, preferably 100 μm or smaller, more preferably50 μm or smaller. The film having a thickness smaller than the aboverange may have insufficient strength as a current collector. Incontrast, the film having a thickness greater than the above range mayhave poor handleability.

In order to reduce the electric contact resistance between the currentcollector and the positive electrode active material layer, the currentcollector also preferably has a conductive aid applied on the surfacethereof. Examples of the conductive aid include carbon and noble metalssuch as gold, platinum, and silver.

The ratio between the thicknesses of the current collector and thepositive electrode active material layer may be any value, and the ratio{(thickness of positive electrode active material layer on one sideimmediately before injection of electrolyte solution)/(thickness ofcurrent collector)} is preferably 20 or lower, more preferably 15 orlower, most preferably 10 or lower. The ratio is also preferably 0.5 orhigher, more preferably 0.3 or higher, most preferably 1 or higher. Thecurrent collector and the positive electrode active material layershowing a ratio higher than the above range may cause the currentcollector to generate heat due to Joule heating duringhigh-current-density charge and discharge. The current collector and thepositive electrode active material layer showing a ratio lower than theabove range may cause an increased ratio by volume of the currentcollector to the positive electrode active material, reducing thebattery capacity.

The positive electrode may be produced by a usual method. An example ofthe production method is a method in which the positive electrode activematerial is mixed with the aforementioned binder, thickening agent,conductive material, solvent, and other components to form a slurry-likepositive electrode mixture, and then this mixture is applied to acurrent collector, dried, and pressed so as to be densified.

The densification may be achieved using a manual press or a roll press,for example. The density of the positive electrode active material layeris preferably 1.5 g/cm³ or higher, more preferably 2 g/cm³ or higher,still more preferably 2.2 g/cm³ or higher, while preferably 5 g/cm³ orlower, more preferably 4.5 g/cm³ or lower, still more preferably 4 g/cm³or lower. The positive electrode active material layer having a densityhigher than the above range may cause low permeability of theelectrolyte solution toward the vicinity of the interface between thecurrent collector and the active material, and poor charge and dischargecharacteristics particularly at a high current density, failing toprovide high output. The positive electrode active material layer havinga density lower than the above range may cause poor conductivity betweenthe active materials and increase the battery resistance, failing toprovide high output.

In order to improve the stability at high output and high temperature inthe case of using the electrolyte solution of the invention, the area ofthe positive electrode active material layer is preferably largerelative to the outer surface area of an external case of the battery.Specifically, the total area of the positive electrode is preferably 15times or more, more preferably 40 times or more, greater than thesurface area of the external case of the secondary battery. For closed,square-shaped cases, the outer surface area of an external case of thebattery herein means the total area calculated from the dimensions oflength, width, and thickness of the case portion into which apower-generating element is packed except for a protruding portion of aterminal. For closed, cylinder-like cases, the outer surface area of anexternal case of the battery herein means the geometric surface area ofan approximated cylinder of the case portion into which apower-generating element is packed except for a protruding portion of aterminal. The total area of the positive electrode herein means thegeometric surface area of the positive electrode mixture layer oppositeto a mixture layer including the negative electrode active material. Forstructures including a current collector foil and positive electrodemixture layers on both sides of the current collector, the total area ofthe positive electrode is the sum of the areas calculated on therespective sides.

The positive electrode plate may have any thickness. In order to achievea high capacity and high output, the lower limit of the thickness of themixture layer on one side of the current collector excluding thethickness of the base metal foil is preferably 10 μm or greater, morepreferably 20 μm or greater, while preferably 500 μm or smaller, morepreferably 450 μm or smaller.

To a surface of the positive electrode plate may be attached a substancehaving a composition different from the positive electrode plate.Examples of the substance attached to the surface include oxides such asaluminum oxide, silicon oxide, titanium oxide, zirconium oxide,magnesium oxide, calcium oxide, boron oxide, antimony oxide, and bismuthoxide; sulfates such as lithium sulfate, sodium sulfate, potassium,sulfate, magnesium sulfate, calcium sulfate, and aluminum sulfate;carbonates such as lithium carbonate, calcium carbonate, and magnesiumcarbonate; and carbon.

<Negative Electrode>

The negative electrode includes a negative electrode active materiallayer containing a negative electrode active material and a currentcollector.

Examples of the negative electrode active material include carbonaceousmaterials that can occlude and release lithium such as pyrolysates oforganic matter under various pyrolysis conditions, artificial graphite,and natural graphite; metal oxide materials that can occlude and releaselithium such as tin oxide and silicon oxide; lithium metals; variouslithium alloys; and lithium-containing metal complex oxide materials.Two or more of these negative electrode active materials may be used inadmixture with each other.

The carbonaceous material that can occlude and release lithium ispreferably artificial graphite produced by high-temperature treatment ofeasily graphitixable pitch from various materials, purified naturalgraphite, or a material obtained by surface treatment on such graphitewith pitch or other organic matter and then carbonization of thesurface-treated graphite. In order to achieve a good balance between theinitial irreversible capacity and the high-current-density charge anddischarge characteristics, the carbonaceous material is more preferablyselected from carbonaceous materials obtained by heat-treating naturalgraphite, artificial graphite, artificial carbonaceous substances, orartificial graphite substances at 400° C. to 3200° C. once or more;carbonaceous materials which allow the negative electrode activematerial layer to include at least two or more carbonaceous mattershaving different crystallinities and/or have an interface between thecarbonaceous matters having the different crystallinities andcarbonaceous materials which allow the negative electrode activematerial layer to have an interface between at least two or morecarbonaceous matters having different orientations. These carbonaceousmaterials may be used alone or in any combination of two or more at anyratio.

Examples of the carbonaceous materials obtained by heat-treatingartificial carbonaceous substances or artificial graphite substances at400° C. to 3200° C. once or more include coal-based coke,petroleum-based coke, coal-based pitch, petroleum-based pitch, and thoseprepared by oxidizing these pitches; needle coke, pitch coke, and carbonmaterials prepared by partially graphitizing these cokes; pyrolysates oforganic matter such as furnace black, acetylene black, and pitch-basedcarbon fibers; carbonizable organic matter and carbides thereof; andsolutions prepared by dissolving carbonizable organic matter in alow-molecular-weight organic solvent such as benzene, toluene, xylene,quinoline, or n-hexane, and carbides thereof.

The metal material, (excluding lithium-titanium, complex oxides) to beused as the negative electrode active material may be any compound thatcan occlude and release lithium, and examples thereof include simplelithium, simple metals and alloys that constitute lithium alloys, andoxides, carbides, nitrides, silicides, sulfides, and phosphides thereof.The simple metals and alloys constituting lithium alloys are preferablymaterials containing any of metal and semi-metal elements in Groups 13and 14, more preferably simple metal of aluminum, silicon, and tin(hereinafter, referred to as “specific metal elements”), and alloys andcompounds containing any of these atoms. These materials may be usedalone or in combination of two or more at any ratio.

Examples of the negative electrode active material containing at leastone atom selected from the specific metal elements include simple metalof any one specific metal element, alloys of two or more specific metalelements, alloys of one or two or more specific metal elements and oneor two or more other metal elements, compounds containing one or two ormore specific metal elements, and composite compounds such as oxides,carbides, nitrides, silicides, sulfides, and phosphides of thecompounds. Such a simple metal, alloy, or metal compound used as thenegative electrode active material can lead to a high-capacity battery.

Examples thereof further include compounds in which any of the abovecomposite compounds are complexly bonded with several elements such assimple metals, alloys, and nonmetal elements. Specifically, in the caseof silicon or tin, for example, an alloy of this element and a metalthat does not serve as a negative electrode may be used. In the case oftin, for example, a composite compound including a combination of 5 or 6elements, including tin, a metal (excluding silicon) that serves as anegative electrode, a metal that does not serve as a negative electrode,and a nonmetal element, may be used.

Specific examples thereof include simple Si, SiB₄, SiB₆, Mg₂Si, Ni₂Si,TiSi₂, MoSi₂, CoSi₂, NiSi₂, CaSi₂, CrSi₂, Cu₆Si, FeSi₂, MnSi₂, NbSi₂,TaSi₂, VSi₂, WSi₂, ZnSi₂, SiC, Si₃N₄, Si₂N₂O, SiO_(v) (0<v≤2) , LiSiO,simple tin, SnSiO₃, LiSnO, Mg₂Sn, and SnO_(w) (0<w≤2).

Examples thereof further include composite materials of Si or Sn used asa first constitutional element, and second and third constitutional,elements. The second constitutional element is at least one selectedfrom cobalt, iron, magnesium, titanium, vanadium, chromium, manganese,nickel, copper, zinc, gallium, and zirconium, for example. The thirdconstitutional element is at least one selected from boron, carbon,aluminum, and phosphorus, for example.

In order to achieve a high battery capacity and excellent batterycharacteristics, the metal material is preferably simple silicon or tin(which may contain trace impurities), SiOv (0<v≤2), SnOw (0≤w≤2), aSi—Co—C composite material, a Si—Ni—C composite material, a Sn—Co—Ccomposite material, or a Sn—Ni—C composite material.

The lithium-containing metal complex oxide material to be used as thenegative electrode active material may be any material that can occludeand release lithium. In order to achieve good high-current-densitycharge and discharge characteristics, materials containing titanium andlithium are preferred, lithium-containing metal complex oxide materialscontaining titanium are more preferred, and complex oxides of lithiumand titanium (hereinafter, abbreviated as “lithium titanium complexoxides”) are still more preferred. In other words, use of aspinel-structured lithium titanium complex oxide in the negativeelectrode active material for an electrolyte battery is particularlypreferred because this can markedly reduce the output resistance.

Preferred examples of the lithium titanium complex oxides includecompounds represented by the following formula:

Li_(x)Ti_(y)M_(z)O₄

wherein M is at least one element selected from the group consisting ofNa, K, Co, Al, Fe, Ti, Mg, Cr, Ga, Cu, Zn, and Nb.

In order to achieve a good balance of the battery performance,particularly preferred among the above compositions are those satisfyingany of the following;

1.2≤x≤1.4, 1.5≤y≤1.7, z=0   (i)

0.9≤x≤1.1, 1.9≤y≤2.1, z=0   (ii)

0.7≤x≤0.9, 2.1≤y≤2.3, z=0.   (iii)

Particularly preferred representative composition of the compound isLi_(4/3)Ti_(5/3)O₄ corresponding to the composition (i), Li₁Ti₂O₄corresponding to the composition (ii), and Li_(4/5)Ti_(11/5)O₄corresponding to the composition (iii). Preferred examples of thestructure satisfying Z≠0 include Li_(4/3)Ti_(4/3)Al_(1/3)O₄.

The negative electrode mixture preferably further contains a binder, athickening agent, and a conductive material.

Examples of the binder include the same binders as those mentioned forthe positive electrode. The proportion of the binder is preferably 0.1%by mass or more, more preferably 0.5% by mass or more, particularlypreferably 0.6% by mass or more, while preferably 20% by mass or less,more preferably 15% by mass or less, still more preferably 10% by massor less, particularly preferably 8% by mass or less, relative to thenegative electrode active material. The binder at a proportion relativeto the negative electrode active material higher than the above rangemay lead to an increased proportion of the binder which fails tocontribute to the battery capacity, causing a low battery capacity. Thebinder at a proportion lower than the above range may cause loweredstrength of the negative electrode.

In particular, in the case of using a rubbery polymer typified by SBR asa main component, the proportion of the binder is usually 0.1% fay massor more, preferably 0.5% by mass or more, more preferably 0.6% by massor more, while usually 5% by mass or less, preferably 3% by mass orless, more preferably 2% by mass or less, relative to the negativeelectrode active material. In the case of using a fluoropolymer typifiedby polyvinylidene fluoride as a main component, the proportion of thebinder is usually 1% by mass or more, preferably 2% by mass or more,more preferably 3% by mass or more, while usually 15% by mass or less,preferably 10% by mass or less, more preferably 8% by mass or less,relative to the negative electrode active material.

Examples of the thickening agent include the same thickening agents asthose mentioned for the positive electrode. The proportion of thethickening agent is usually 0.1% by mass or higher, preferably 0.5% bymass or higher, still more preferably 0.6% by mass or higher, whileusually 5% by mass or lower, preferably 3% by mass or lower, still morepreferably 2% by mass or lower, relative to the negative electrodeactive material. The thickening agent at a proportion relative to thenegative electrode active material lower than the above range may causesignificantly poor easiness of application. The thickening agent at aproportion higher than the above range may cause a small proportion ofthe negative electrode active material in the negative electrode activematerial layer, resulting in a low capacity of the battery and highresistance between the negative electrode active materials.

Examples of the conductive material of the negative electrode includemetal materials such as copper and nickel; and carbon materials such asgraphite and carbon black.

The solvent for forming slurry may be any solvent that can dissolve ordisperse the negative electrode active material and the binder, as wellas a thickening agent and a conductive material used as appropriate. Thesolvent may be either an aqueous solvent or an organic solvent.

Examples of the aqueous solvent include water and alcohols. Examples ofthe organic solvent include N-methylpyrrolidone (NMP), dimethylformamide, dimethyl acetamide, methyl ethyl ketone, cyclohexanone,methyl acetate, methyl acrylate, diethyl triamine, N,N-dimethylaminopropyl amine, tetrahydrofuran (THF), toluene, acetone, diethylether, dimethyl acetamide, hexamethyl phospharamide, dimethyl sulfoxide,benzene, xylene, quinoline, pyridine, methyl naphthalene, and hexane.

Examples of the material of the current collector for a negativeelectrode include copper, nickel, and stainless steel. In order toeasily process the material into a film and to minimize the cost, copperis preferred.

The current collector usually has a thickness of 1 μm or greater,preferably 5 μm or greater, while usually 100 μm or smaller, preferably50 μm or smaller. Too thick a negative electrode current collector maycause an excessive reduction in capacity of the whole battery, while toothin a current collector may be difficult to handle.

The negative electrode may be produced by a usual method. An example ofthe production method is a method in which the negative electrodematerial, is mixed with the aforementioned, binder, thickening agent,conductive material, solvent, and other components to form a slurry-likemixture, and then this mixture is applied to a current collector, dried,and pressed so as to be densified. In the case of using an alloyedmaterial, a thin film layer containing the above negative electrodeactive material (negative electrode active material layer) may beproduced by vapor deposition, sputtering, plating, or the like.

The electrode formed from the negative electrode active material mayhave any structure. The negative electrode active material existing onthe current collector preferably has a density of 1 g·cm⁻³ or higher,more preferably 1.2 g·cm⁻³ or higher, particularly preferably 1.3 g·cm⁻³or higher, while preferably 2.2 g·cm⁻³ or lower, more preferably 2.1g·cm⁻³ or lower, still more preferably 2.0 g·cm⁻³ or lower, particularlypreferably 1.9 g·cm⁻³ or lower. The negative electrode active materialexisting on the current collector having a density higher than the aboverange may cause destruction of the negative electrode active materialparticles, resulting in a high initial irreversible capacity and poorhigh-current-density charge and discharge characteristics due toreduction in permeability of the electrolyte solution toward thevicinity of the interface between the current collector and the negativeelectrode active material. The negative electrode active material havinga density below the above range may cause poor conductivity between thenegative electrode active materials, high battery resistance, and a lowcapacity per unit volume.

The thickness of the negative electrode plate is a design matter inaccordance with the positive electrode plate to be used, and may be anyvalue. The thickness of the mixture layer excluding the thickness of thebase metal foil is usually 15 μm or greater, preferably 20 μm orgreater, more preferably 30 μm or greater, while usually 300 μm orsmaller, preferably 280 μm or smaller, more preferably 250 μm orsmaller.

To a surface of the negative electrode plate may be attached a substancehaving a composition different from the negative electrode plate.Examples of the substance attached to the surface include oxides such asaluminum oxide, silicon oxide, titanium oxide, zirconium oxide,magnesium oxide, calcium oxide, boron oxide, antimony oxide, and bismuthoxide; sulfates such as lithium sulfate, sodium sulfate, potassiumsulfate, magnesium sulfate, calcium sulfate, and aluminum sulfate; andcarbonates such as lithium carbonate, calcium carbonate, and magnesiumcarbonate.

<Separator>

The lithium-ion secondary battery preferably further includes aseparator.

The separator may be formed from any known material and may have anyknown shape as long as the resulting separator is stable to theelectrolyte solution and is excellent in a liquid-retaining ability. Theseparator is preferably in the form of a porous sheet or a nonwovenfabric which is formed from a material stable to the electrolytesolution of the invention, such as resin, glass fiber, or inorganicmatter, and which has an excellent liquid-retaining ability.

Examples of the material of a resin or glass-fiber separator includepolyolefins such as polyethylene and polypropylene, aromatic polyamide,polytetrafluoroethylene, polyether sulfone, and glass filters. Thesematerials may be used alone or in any combination of two or more at anyratio, for example, in the form of a polypropylene/polyethylene bilayerfilm or a polypropylene/polyethylene/polypropylene trilayer film. Inorder to achieve good permeability of the electrolyte solution and agood shut-down effect, the separator is preferably a porous sheet or anonwoven fabric formed from a polyolefin such as polyethylene orpolypropylene.

The separator may have any thickness, and the thickness is usually 1 μmor greater, preferably 5 μm or greater, more preferably 8 μm or greater,while usually 50 μm or smaller, preferably 40 μm or smaller, morepreferably 30 μm or smaller. The separator thinner than the above rangemay have poor insulation and mechanical strength. The separator thickerthan the above range may cause not only poor battery performance such aspoor rate characteristics but also a low energy density of the wholeelectrolyte battery.

The separator which is a porous one such as a porous sheet or a nonwovenfabric may have any porosity. The porosity is usually 20% or higher,preferably 35% or higher, more preferably 45% or higher, while usually90% or lower, preferably 85% or lower, more preferably 75% or lower. Theseparator having a porosity lower than the above range tends to havehigh film resistance, causing poor rate characteristics. The separatorhaving a porosity higher than the above range tends to have lowmechanical strength, causing poor insulation.

The separator may also have any average pore size. The average pore sizeis usually 0.5 μm or smaller, preferably 0.2 μm or smaller, whileusually 0.05 μm or larger. The separator having an average pore sizelarger than the above range may easily cause short circuits. Theseparator having an average pore size smaller than the above range mayhave high film resistance, causing poor rate characteristics.

Examples of the inorganic matter include oxides such as alumina andsilicon dioxide, nitrides such as aluminum nitride and silicon nitride,and sulfates such as barium sulfate and calcium sulfate, each in theform of particles or fibers.

The separator is in the form of a thin film such as a nonwoven fabric, awoven fabric, or a microporous film. The thin film favorably has a poresize of 0.01 to 1 μm and a thickness of 5 to 50 μm. Instead of the aboveseparate thin film, the separator may have a structure in which acomposite porous layer containing particles of the above inorganicmatter is disposed on a surface of one or each of the positive andnegative electrodes using a resin binder. For example, alumina particleshaving a 90% particle size of smaller than 1 μm may be applied to therespective surfaces of the positive electrode with fluororesin used as abinder to form a porous layer.

<Battery Design>

The electrode group may be either a laminate structure including theabove positive and negative electrode plates with the above separator inbetween, or a wound structure including the above positive and negativeelectrode plates in spiral with the above separator in between. Theproportion of the volume of the electrode group in the battery internalvolume (hereinafter, referred to as an electrode group proportion) isusually 40% or higher, preferably 50% or higher, while usually 90% orlower, preferably 80% or lower.

The electrode group proportion lower than the above range may cause alow battery capacity. The electrode group proportion higher than theabove range may cause small void space in the battery. Thus, if thebattery temperature rises to high temperature and thereby the componentsswell and the liquid fraction of the electrolyte solution exhibits highvapor pressure to raise the internal pressure, the batterycharacteristics such as charge and discharge repeatability andhigh-temperature storageability may be impaired and a gas-releasingvalve for releasing the internal pressure toward the outside may beactuated.

The current collecting structure may be any structure. In order to moreeffectively improve the high-current-density charge and dischargeperformance by the electrolyte solution of the invention, the currentcollecting structure is preferably a structure which reduces theresistances at wiring portions and jointing portions. Such reduction ininternal resistance can particularly favorably lead to the effectsachieved with the electrolyte solution of the invention.

In an electrode group having the laminate structure, the metal coreportions of the respective electrode layers are preferably bundled andwelded to a terminal. If an electrode has a large area, the internalresistance is high. Thus, multiple terminals may preferably be disposedin the electrode so as to reduce the resistance. In an electrode grouphaving the wound structure, multiple lead structures may be disposed oneach of the positive electrode and the negative electrode and bundled toa terminal. This can reduce the internal resistance,

The external case may be made of any material that is stable to anelectrolyte solution to be used. Specific examples thereof includemetals such as nickel-plated steel plates, stainless steel, aluminum andaluminum alloys, and magnesium alloys, and a layered film (laminatefilm) of resin and aluminum foil. In order to reduce the weight, a metalsuch as aluminum or an aluminum alloy or a laminate film is favorablyused.

An external case made of metal may have a sealed-up structure formed bywelding the metal by laser welding, resistance welding, or ultrasonicwelding, or a caulking structure using the metal with a resin gasket inbetween. An external, case made of a laminate film may have a sealed-upstructure formed by hot-melting resin layers. In order to improve thesealability, a resin which is different from the resin of the laminatefilm may be disposed between the resin layers. Especially, in the caseof forming a sealed-up structure by hot-melting the resin layers withcurrent collecting terminals in between, metal and resin are to bebonded. Thus, the resin to be disposed between the resin layers isfavorably a resin having a polar group or a modified resin having apolar group introduced therein.

The lithium-ion secondary battery may have any shape, such as acylindrical shape, a square shape, a laminate shape, a coin shape, or alarge-size shape. The shapes and the structures of the positiveelectrode, the negative electrode, and the separator may be changed inaccordance with the shape of the battery.

A module including the secondary battery is also one aspect of theinvention.

In a preferred embodiment, the secondary battery includes a positiveelectrode, a negative electrode, and the aforementioned electrolytesolution, the positive electrode including a positive electrode currentcollector and a positive electrode active material layer containing apositive electrode active material, the positive electrode activematerial containing Mn. The secondary battery including a positiveelectrode active material layer that contains a positive electrodeactive material containing Mn can have much better high-temperaturestorage characteristics.

In order to provide a high-power secondary battery having a high energydensity, preferred as the positive electrode active material containingMn are LiMn_(1.5)Ni_(0.5)O₄, LiNi_(0.5)Co_(0.2)Mn_(0.3)O₂, andLiNi_(0.6)Co_(0.2)Mn_(0.2)O₂.

The amount of the positive electrode active material in the positiveelectrode active material layer is preferably 80% by mass or more, morepreferably 82% by mass or more, particularly preferably 84% by mass ormore. The upper limit of the amount thereof is preferably 99% by mass orless, more preferably 98% by mass or less. Too small an amount of thepositive electrode active material in the positive electrode activematerial layer may lead to an insufficient electric capacity. Incontrast, too large an amount thereof may lead to insufficient strengthof the positive electrode.

The positive electrode active material layer may further contain aconductive material, a thickening agent, and a binder.

The binder may be any material that is safe against a solvent to be usedin production of electrodes and the electrolyte solution. Examplesthereof include polyvinylidene fluoride, polytetrafluoroethylene,polyethylene, polypropylene, SBR (styrene-butadiene rubber), isoprenerubber, butadiene rubber, ethylene-acrylic acid copolymers,ethylene-methacrylic acid copolymers, polyethylene terephthalate,polymethyl methacrylate, polylmide, aromatic polyamide, cellulose, nitrocellulose, NBR (acrylonitrile-butadiene rubber), fluoroelastomer,ethylene-propylene rubber, styrene-butadiene-styrene block copolymersand hydrogenated products thereof, EPDM (ethylene-propylene-dieneterpolymers), styrene-ethylene-butadiene-ethylene copolymers,styrene-isoprene-styrene block copolymers and hydrogenated productsthereof, syndiotactic-1,2-polybutadiene, polyvinyl acetate,ethylene-vinyl acetate copolymers, propylene-α-olefin copolymers,fluorinated polyvinylidene fluoride, tetrafluoroethylene-ethylenecopolymers, and polymer compositions having ion conductivity of alkalimetal ions (especially, lithium ions). These substances may be usedalone or in any combination of two or more at any ratio.

The amount of the binder, which is expressed as the proportion of thebinder in the positive electrode active material layer, is usually 0.1%by mass or more, preferably 1% by mass or more, more preferably 1.5% bymass or more. The proportion is also usually 80% by mass or less,preferably 60% by mass or less, still more preferably 40% by mass orless, most preferably 10% by mass or less. Too low a proportion of thebinder may fail to sufficiently hold the positive electrode activematerial and cause insufficient mechanical strength of the positiveelectrode, impairing the battery performance such as cyclecharacteristics. In contrast, too high a proportion thereof may causereduction in battery capacity and conductivity.

Examples of the thickening agent include carboxymethyl cellulose, methylcellulose, hydroxymethyl cellulose, ethyl cellulose, polyvinyl alcohol,oxidized starch, monostarch phosphate, casein, and salts thereof. Theseagents may be used alone or in any combination of two or more at anyratio.

The proportion of the thickening agent relative to the active materialis usually 0.1% by mass or higher, preferably 0.2% by mass or higher,more preferably 0.3% by mass or higher, while usually 5% by mass orlower, preferably 3% by mass or lower, more preferably 2% by mass orlower. The thickening agent at a proportion lower than the above rangemay cause significantly poor easiness of application. The thickeningagent at a proportion higher than the above range may cause a lowproportion of the active material in the positive electrode activematerial layer, resulting in a low capacity of the battery and highresistance between the positive electrode active materials.

The conductive material may be any known conductive material. Specificexamples thereof include metal materials such as copper and nickel, andcarbon materials such as graphite, including natural graphite andartificial graphite, carbon black, including acetylene black, andamorphous carbon, including needle coke. These materials may be usedalone or in any combination of two or more at any ratio. The conductivematerial is used in an amount of usually 0.01% by mass or more,preferably 0.1% by mass or more, more preferably 1% by mass or more,while usually 50% by mass or less, preferably 30% by mass or less, morepreferably 15% by mass or less, in the positive electrode activematerial layer. The conductive material in an amount less than the aboverange may cause insufficient conductivity. In contrast, conductivematerial in an amount more than the above range may cause a low batterycapacity.

In order to further improve the high-temperature storagecharacteristics, the positive electrode current collector is preferablyformed from a valve metal or an alloy thereof. Examples of the valvemetal include aluminum, titanium, tantalum, and chromium. The positiveelectrode current collector is more preferably formed from aluminum oran alloy of aluminum.

In order to further improve the high-temperature storage characteristicsof the secondary battery, a portion in contact with the electrolytesolution among portions electrically coupled with the positive electrodecurrent collector is also preferably formed from a valve metal or analloy thereof. In particular, the external case of the battery and aportion that is electrically coupled with the positive electrode currentcollector and is in contact with the non-aqueous electrolyte solutionamong components accommodated in the external case of the battery, suchas leads and a safety valve, are preferably formed from a valve metal oran alloy thereof. Stainless steel coated with a valve metal or an alloythereof may also be used.

The positive electrode may be produced by the aforementioned method. Anexample of the production method is a method in which the positiveelectrode active material is mixed with the aforementioned binder,thickening agent, conductive material, solvent, and other components toform a slurry-like positive electrode mixture, and then this mixture isapplied to a positive electrode current collector, dried, and pressed soas to be densified.

The structure of the negative electrode is as described above.

The electric double-layer capacitor may include a positive electrode, anegative electrode, and the aforementioned electrolyte solution.

At least one selected from the positive electrode and the negativeelectrode is a polarizable electrode in the electric double-layercapacitor. Examples of the polarizable electrode and a non-polarizableelectrode include the following electrodes specifically disclosed in JPH09-7896 A.

The polarizable electrode mainly containing activated carbon to be usedin the invention preferably contains inactivated carbon having a largespecific surface area and a conductive material, such as carbon black,providing electronic conductivity. The polarizable electrode may beformed by a variety of methods. For example, a polarizable electrodeincluding activated carbon and carbon black can be produced by mixingactivated carbon powder, carbon black, and phenolic resin, press-moldingthe mixture, and then sintering and activating the mixture in an inertgas atmosphere and water vapor atmosphere. Preferably, this polarizableelectrode is bonded to a current collector using a conductive adhesive,for example.

Alternatively, a polarizable electrode can also be formed by kneadingactivated carbon powder, carbon black, and a binder in the presence ofan alcohol, forming the mixture into a sheet, and then drying the sheet.The binder to be used may be polytetrafluoroethylene, for example.Alternatively, a polarizable electrode integrated with a currentcollector can be produced by mixing activated carbon powder, carbonblack, a binder, and a solvent to form slurry, applying this slurry tometal foil of a current collector, and then drying the slurry.

The electric double-layer capacitor may have polarizable electrodesmainly containing activated carbon as the respective electrodes. Still,the electric double-layer capacitor may have a structure in which anon-polarizable electrode is used on one side. Examples of such astructure include a structure in which a positive electrode mainlycontaining an electrode active material such as a metal oxide iscombined with a polarisable negative electrode mainly containingactivated carbon; and a structure in which a negative electrode mainlycontaining a carbon material that can reversibly occlude and releaselithium ions or a negative electrode of lithium metal or lithium alloyis combined with a polarizable positive electrode mainly containingactivated carbon.

In place of or in combination with activated carbon, any carbonaceousmaterial may be used, such as carbon black, graphite, expanded graphite,porous carbon, carbon nanotube, carbon nanohorn, and Ketjenblack.

The non-polarizable electrode is preferably an electrode mainlycontaining a carbon material that can reversibly occlude and releaselithium ions, with this carbon material made to occlude lithium ions inadvance. In this case, the electrolyte used is a lithium salt. The selectric double-layer capacitor having such a structure can achieve amuch higher withstand voltage exceeding 4 V,

The solvent used in preparation of the slurry in production ofelectrodes is preferably one that dissolves a binder. In accordance withthe type of a binder, the solvent is appropriately selected fromN-methylpyrrolidone, dimethyl formamide, toluene, xylene, isophorone,methyl ethyl ketone, ethyl acetate, methyl acetate, dimethyl phthalate,ethanol, methanol, butanol, and water.

Examples of the activated carbon used for the polarizable electrodeinclude phenol resin-type activated carbon, coconut shell-type activatedcarbon, and petroleum coke-type activated carbon. In order to achieve alarge capacity, petroleum coke-type activated carbon or phenolresin-type activated carbon is preferably used. Examples of methods ofactivating the activated carbon include steam activation and molten KOHactivation. In order to achieve a larger capacity, activated carbonprepared fay molten KOH activation is preferably used.

Preferred examples of the conductive agent used for the polarizableelectrode include carbon black, Ketjenblack, acetylene black, naturalgraphite, artificial graphite, metal fiber, conductive titanium oxide,and ruthenium oxide. In order to achieve good conductivity (i.e., lowinternal resistance), and because too large an amount thereof may leadto a decreased capacity of the product, the amount of the conductiveagent such as carbon black used for the polarizable electrode ispreferably 1 to 50% by mass in the sum of the amounts of the activatedcarbon and the conductive agent.

In order to provide an electric double-layer capacitor having a largecapacity and low internal resistance, the activated carbon used for thepolarizable electrode preferably has an average particle size of 20 μmor smaller and a specific surface area of 1500 to 3000 m²/g. Preferredexamples of the carbon material for providing an electrode mainlycontaining a carbon material that can reversibly occlude and releaselithium ions include natural graphite, artificial graphite, graphitizedmesocarbon microsphere, graphitized whisker, vapor-grown carbon fiber,sintered furfuryl alcohol resin, and sintered novolak resin.

The current collector may be any chemically and electrochemicallycorrosion-resistant one. Preferred examples of the current collectorused for the polarizable electrode mainly containing activated carboninclude stainless steel, aluminum, titanium, and tantalum. Particularlypreferred materials in terms of the characteristics and cost of theresulting electric double-layer capacitor are stainless steel andaluminum. Preferred examples of the current collector used for theelectrode mainly containing a carbon material that can reversiblyocclude and release lithium-ions include stainless steel, copper, andnickel.

Examples of methods of allowing the carbon material that can reversiblyocclude and release lithium ions to occlude lithium ions in advanceinclude: (1) a method of mixing powdery lithium to a carbon materialthat can reversibly occlude and release lithium ions; (2) a method ofplacing lithium foil on an electrode including a carbon material thatcan reversibly occlude and release lithium ions and a binder so as tobring the lithium foil to be in electrical contact with the electrode,immersing this electrode In an electrolyte solution containing a lithiumsalt dissolved therein so as to ionize the lithium, and allowing thecarbon material to take in the lithium ions; and (3) a method of placingan electrode including a carbon material that can reversibly occlude andrelease lithium ions and a binder on the minus side and placing alithium metal on the plus side, immersing the electrodes in anelectrolyte solution containing a lithium salt as an electrolyte, andsupplying a current so that the carbon material is allowed toelectrochemically take in the ionized lithium.

Examples of known electric double-layer capacitors include woundelectric double-layer capacitors, laminated electric double-layercapacitors, and coin-type electric double-layer capacitors. The electricdouble-layer capacitor may also be any of these types.

For example, a wound electric double-layer capacitor may be assembled asfollows. A positive electrode and a negative electrode each of whichincludes a laminate (electrode) of a current collector and an electrodelayer are wound with a separator in between to provide a wound element.This wound element is put into a case made of aluminum, for example. Thecase is filled, with an electrolyte solution, preferably a non-aqueouselectrolyte solution, and then sealed with a rubber sealant.

A separator formed from a conventionally known material and having aconventionally known structure may be used. Examples thereof includepolyethylene porous membranes and nonwoven fabric of polypropylenefiber, glass fiber, or cellulose fiber.

In accordance with any known method, the electric double-layer capacitormay be prepared in the form of a laminated electric double-layercapacitor in which sheet-like positive and negative electrodes arestacked with an electrolyte solution and a separator in between or acoin-type electric double-layer capacitor in which positive and negativeelectrodes are fixed in a coin shape by a gasket with an electrolytesolution and a separator in between.

The electrolyte solution of the invention is useful as an electrolytesolution for large-size lithium-ion secondary batteries for hybridvehicles or distributed generation, and for electric double-layercapacitors.

EXAMPLES

The invention is described with reference to examples, but the examplesare not intended to limit the invention.

The abbreviations in Table 1 and Table 2 represent the followingcompounds. The blending proportions of the solvents in Table 1 and Table2 are volume proportions (% by volume) of the components in the solvent,and the blending proportions of the additives are mass proportions (% bymass) of the additives relative to the mass of the solvent in theelectrolyte solution.

A: ethylene carbonate

B: ethyl methyl carbonate

C; dimethyl carbonate

D: ethyl propionate

E: fluoroethylene carbonate

F: vinylene carbonate

G: CH₃OCOOCH₂CF₃

H: CHF₂COOCH₃

-   I: CF₃CH₂COOCH₃

J: tris(trimethylsilyl) phosphite

K: trifluoromethylethylene carbonate

L: 4-(2,2,3,3,3-pentafluoropropyl)1,3-dioxolan-2-one

M: 4-(2,3,3,3-tetrafluoro-2-trifluoromethylpropyl)1,3-dioxolan-2-one

Examples and Comparative Examples in Table 1

The components were mixed at proportions shown in Table 1, LiPF₆ wasadded thereto so as to be at a concentration of 1.0 mol/L. Thereby, anon-aqueous electrolyte solution was obtained.

(Production 1 of Positive Electrode)

LiNi_(1/3)Mn_(1/3)Co_(1/3)O₂ serving as a positive electrode activematerial, acetylene black serving as a conductive material, and adispersion of polyvinylidene fluoride (PVdF) in N-methyl-2-pyrrolidoneserving as a binding agent were mixed in a solids content ratio ofactive material/conductive material/binding agent=92/3/5 (ratio by % bymass). Thereby, positive electrode mixture slurry was prepared. Theresulting positive electrode mixture slurry was uniformly applied to a20-μm-thick aluminum foil current collector and dried. The workpiece wasthen compression molded, whereby a positive electrode was produced.

(Production of Negative Electrode)

Artificial graphite powder serving as a negative electrode activematerial, an aqueous dispersion of sodium carboxymethyl cellulose(concentration of sodium carboxymethyl cellulose: 1% by mass) serving asa thickening agent, and an aqueous dispersion of styrene-butadienerubber (concentration of styrene-butadiene rubber: 50% by mass) servingas a binding agent were mixed in an aqueous solvent in a solids contentratio of active material/thickening agent/binding agent=97.6/1.2/1.2(ratio by % by mass) to give a mixture in the form of slurry. Thereby,negative electrode mixture slurry was prepared. The negative electrodemixture slurry was uniformly applied to a 20-μm-thick copper foil anddried. The workpiece was then compressed using a press, whereby anegative electrode was produced.

(Production of Lithium Ion Secondary Battery)

The negative electrode and the positive electrode produced as describedabove and a polyethylene separator were stacked in the order of thenegative electrode, the separator, and the positive electrode. Thereby,a battery element was produced.

A bag of a laminate film, which was formed by covering each surface ofan aluminum sheet (thickness: 40 μm) with a resin layer, was preparedand the battery element was inserted into the bag with terminals of thepositive electrode and the negative electrode protruding therefrom. Thebag was charged with an electrolyte solution having the compositionshown in Table 1 and sealed in vacuo. Thereby, a sheet-shaped lithiumion secondary battery was produced.

(Measurement of Resistance Increase)

The battery was charged up to a SOC of 50% and discharged at a 5 C rate.The voltage drop (V) during the process was divided by the correspondingcurrent value (A), whereby the IV resistance (Q) was calculated. Then,the battery was charged up to 4.2 V and stored in an open-circuit stateat 60° C. for four weeks. The stored battery was adjusted to a SOC of50% and discharged in the above state at a 5 C rate. The voltage drop(V) during the process was divided by the corresponding current value(A), whereby the IV resistance (Ω) was calculated. In each example, theratio of the values before storage and after storage was calculated. Thevalue in Example 4 was taken as 100, and was compared with the values ofthe respective examples.

The term SOC means the state of charge, and a SOC of 100% means thecapacity of the battery charged up to 4.2 V. For example, a SOC of 50%means half the capacity of a SOC of 100%.

Table 1 shows the results.

TABLE 1 Solvent Additive Blending Blending Blending Blending Blendingproportion proportion proportion proportion proportion Resistance Type(vol %) Type (vol %) Type (mass %) Type (mass %) Type (mass %) IncreaseExample 1 A 30 B 70 J 0.001 L 0.5 — 0 106 Example 2 A 30 B 70 J 0.01 L0.5 — 0 104 Example 3 A 30 B 70 J 0.1 L 0.5 — 0 103 Example 4 A 30 B 70J 0.5 L 0.5 — 0 100 Example 5 A 30 B 70 J 1 L 0.5 — 0 108 Example 6 A 30B 70 J 10 L 0.5 — 0 107 Example 7 A 30 B 70 J 0.5 L 0.001 — 0 105Example 8 A 30 B 70 J 0.5 L 0.01 — 0 104 Example 9 A 30 B 70 J 0.5 L 0.1— 0 103 Example 10 A 30 B 70 J 0.5 L 1 — 0 102 Example 11 A 30 B 70 J0.5 L 10 — 0 105 Example 12 A 30 B 70 J 0.5 L 0.5 F 0.001 92 Example 13A 30 B 70 J 0.5 L 0.5 F 0.01 91 Example 14 A 30 B 70 J 0.5 L 0.5 F 0.190 Example 15 A 30 B 70 J 0.5 L 0.5 F 0.5 89 Example 16 A 30 B 70 J 0.5L 0.5 F 1 90 Example 17 A 30 B 70 J 0.5 L 0.5 F 10 93 Example 18 A 30 B70 J 0.5 M 0.5 — 0 101 Example 19 A 30 B 70 J 0.5 M 0.5 F 0.5 90 Example20 A 30 C 70 J 0.5 L 0.5 — 0 102 Example 21 A 30 C 70 J 0.5 L 0.5 F 0.591 Example 22 A 30 D 70 J 0.5 L 0.5 — 0 103 Example 23 A 30 D 70 J 0.5 L0.5 F 0.5 92 Comparative Example 1 A 30 B 70 J 0.5 L 0 — 0 124Comparative Example 2 A 30 B 70 J 0.5 L 0 F 0.5 119 Comparative Example3 A 30 B 70 — 0 L 0.5 — 0 125 Comparative Example 4 A 30 B 70 — 0 L 0.5F 0.5 120

Examples and Comparative Examples in Table 2

The components were mixed at proportions shown in Table 2, LiPF₆ wasadded thereto so as to be at a concentration of 1.0 mol/L. Thereby, anon-aqueous electrolyte solution was obtained.

(Production 2 of Positive Electrode)

LiNi_(0.5)Mn_(1.5)O₄ serving as a positive electrode active material,acetylene black serving as a conductive material, and a dispersion ofpolyvinylidene fluoride (PVdF) in methyl-2-pyrrolidone serving as abinding agent were mixed in a solids content ratio of activematerial/conductive material/binding agent=92/3/5 (ratio by % by mass).Thereby, positive electrode mixture slurry was prepared. The resultingpositive electrode mixture slurry was uniformly applied to a 20-μm-thickaluminum foil current collector and dried. The workpiece was thencompression molded, whereby a positive electrode was produced.

(Production of Negative Electrode)

Artificial graphite powder serving as a negative electrode activematerial, an aqueous dispersion of sodium carboxymethyl cellulose(concentration of sodium carboxymethyl cellulose: 1% by mass) serving asa thickening agent, and an aqueous dispersion of styrene-butadienerubber (concentration of styrene-butadiene rubber: 50% by mass) servingas a binding agent, were mixed in an aqueous solvent in a solids contentratio of active material/thickening agent/binding agent=97.6/1.2/1.2(ratio by % by mass) to give a mixture in the form of slurry. Thereby,negative electrode mixture slurry was prepared. The negative electrodemixture slurry was uniformly applied to a 20-μm-thick copper foil anddried. The workpiece was then compressed using a press, whereby anegative electrode was produced.

(Production of Lithium Ion Secondary Battery)

The negative electrode and the positive electrode produced as describedabove and a polyethylene separator were stacked in the order of thenegative electrode, the separator, and the positive electrode. Thereby,a battery element was produced.

A bag of a laminate film, which was formed by covering each surface ofan aluminum sheet (thickness: 40 μm) with a resin layer, was preparedand the battery element was inserted into the bag with terminals of thepositive electrode and the negative electrode protruding therefrom. Thebag was charged with an electrolyte solution having the compositionshown in Table 2 and sealed in vacuo. Thereby, a sheet-shaped lithiumion secondary battery was produced.

(Measurement of Resistance Increase)

The battery was adjusted up to a SOC of 50% and discharged at a 5 Crate. The voltage drop (V) during the process was divided by thecorresponding current value (A), whereby the IV resistance (Q) wascalculated. Then, the battery was charged up to 4.9 V and stored in anopen-circuit state at 60° C. for four weeks. The stored battery wasadjusted to a SOC of 50% and discharged in the above state at a 5 Crate. The voltage drop (V) during the process was divided by thecorresponding current value (A), whereby the IV resistance (Q) wascalculated. In each example, the ratio of the values before storage andafter storage was calculated. The value in Example 27 was taken as 100,and was compared with the values of the respective examples.

The term SOC means the state of charge, and a SOC of 100% means thecapacity of the battery charged up to 4.9 V. For example, a SOC of 50%means half the capacity of a SOC of 100%.

Table 2 shows the results.

TABLE 2 Solvent Additive Blending Blending Blending Blending Blendingproportion proportion proportion proportion proportion Resistance Type(vol %) Type (vol %) Type (mass %) Type (mass %) Type (mass %) IncreaseExample 24 E 30 G 70 J 0.001 L 0.5 — 0 105 Example 25 E 30 G 70 J 0.01 L0.5 — 0 103 Example 26 E 30 G 70 J 0.1 L 0.5 — 0 101 Example 27 E 30 G70 J 0.5 L 0.5 — 0 100 Example 28 E 30 G 70 J 1 L 0.5 — 0 101 Example 29E 30 G 70 J 10 L 0.5 — 0 106 Example 30 E 30 G 70 J 0.5 L 0.001 — 0 106Example 31 E 30 G 70 J 0.5 L 0.01 — 0 105 Example 32 E 30 G 70 J 0.5 L0.1 — 0 102 Example 33 E 30 G 70 J 0.5 L 1 — 0 102 Example 34 E 30 G 70J 0.5 L 10 — 0 106 Example 35 E 30 G 70 J 0.5 L 0.5 F 0.001 86 Example36 E 30 G 70 J 0.5 L 0.5 F 0.01 87 Example 37 E 30 G 70 J 0.5 L 0.5 F0.1 86 Example 38 E 30 G 70 J 0.5 L 0.5 F 0.5 85 Example 39 E 30 G 70 J0.5 L 0.5 F 1 86 Example 40 E 30 G 70 J 0.5 L 0.5 F 10 89 Example 41 E30 G 70 J 0.5 M 0.5 — 0 101 Example 42 E 30 G 70 J 0.5 M 0.5 F 0.5 87Example 43 K 30 G 70 J 0.5 L 0.5 — 0 101 Example 44 K 30 G 70 J 0.5 L0.5 F 0.5 87 Example 45 E 30 H 70 J 0.5 L 0.5 — 0 102 Example 46 E 30 H70 J 0.5 L 0.5 F 0.5 88 Example 47 E 30 I 70 J 0.5 L 0.5 — 0 102 Example48 E 30 I 70 J 0.5 L 0.5 F 0.5 89 Example 49 K 30 H 70 J 0.5 L 0.5 — 0103 Example 50 K 30 H 70 J 0.5 L 0.5 F 0.5 90 Comparative Example 5 E 30G 70 J 0.5 — 0 — 0 136 Comparative Example 6 E 30 G 70 J 0.5 — 0 F 0.5134 Comparative Example 7 E 30 G 70 — 0 L 0.5 — 0 138 ComparativeExample 8 E 30 G 70 — 0 L 0.5 F 0.5 136

1. An electrolyte solution comprising: tris(trimethylsilyl) phosphite; and at least one fluorinated saturated cyclic carbonate (1) selected from the group consisting of a compound (1-a) and a compound (1-b), the compound (1-a) being represented by the following formula:

the compound (1-b) being represented by the following formula:


2. The electrolyte solution according to claim 1, further comprising a solvent.
 3. The electrolyte solution according to claim 2, wherein the solvent contains at least one selected from the group consisting of a non-fluorinated saturated cyclic carbonate, a fluorinated saturated cyclic carbonate other than the fluorinated saturated cyclic carbonate (1), a non-fluorinated acyclic carbonate, a fluorinated acyclic carbonate, a non-fluorinated acyclic ester, and a fluorinated acyclic ester.
 4. The electrolyte solution according to claim 2, wherein the tris(trimethylsilyl) phosphite is present in an amount of 0.001 to 10% by mass relative to the solvent.
 5. The electrolyte solution according to claim 2, wherein the fluorinated saturated cyclic carbonate (1) is present in an amount of 0.001 to 10% by mass relative to the solvent.
 6. The electrolyte solution according to claim 1, further comprising vinylene carbonate.
 7. The electrolyte solution according to claim 6, further comprising a solvent, wherein the vinylene carbonate is present in an amount of 0.001 to 10% by mass relative to the solvent.
 8. The electrolyte solution according to claim 1, further comprising an electrolyte salt.
 9. An electrochemical device comprising the electrolyte solution according to claim
 1. 10. A secondary battery comprising the electrolyte solution according to claim
 1. 11. A module comprising the electrochemical device according to claim 9 or the secondary battery.
 12. A module comprising the secondary battery according to claim
 10. 